Q. "Why do Jehovah's Witnesses not celebrate birthdays?"
A. For the same reasons that God's faithful worshippers during both the pre-Christian period and during the time of the 1st century Christianity DID NOT partake / participate in the custom of annually celebrating birthdays.
I say "annually", due to the fact that God's faithful worshippers during the pre-Christian period, during the time of 1st century Christianity and of today fully recognized and recognize the joy that comes from having a child born and celebrating this blessed event among family and friends (after-all, LIFE is a blessed gift from God).
However, the custom of annually celebrating the day of ones birth is an entirely different matter altogether and the faithful worshippers of God during the pre-Christian period, during the time of 1st century Christianity and of those of today fully recognized and indeed recognize this.
“The notion of a birthday festival was far from the ideas of the Christians of this period in general.”
—The History of the Christian Religion and Church, During the Three First Centuries (New York, 1848), Augustus Neander (translated by Henry John Rose), p. 190.
“The later Hebrews looked on the celebration of birthdays as a part of idolatrous worship, a view which would be abundantly confirmed by what they saw of the common observances associated with these days.”
—The Imperial Bible-Dictionary (London, 1874), edited by Patrick Fairbairn, Vol. I, p. 225.
There are in actuality 4 main reasons as to why we who are Jehovah's modern day Witnesses DO NOT partake in the custom of annually celebrating birthdays (our own and or anyone else's).
#1. The Holy Scriptures themselves DO NOT put the practice in a favorable light by THE Almighty God whatsoever. There are but only TWO instances of the custom of annual celebration of birthdays recorded in the Bible and it is of great noteworthiness to seekers of God's TRUTH (be such ones members of Jehovah's Witnesses or not) that on BOTH occasions THOSE WHO PARTICIPATED WERE NOT GOD'S FAITHFUL WORSHIPPERS and the events of the celebration were obviously unrighteous.
#2. The custom of ANNUALLY celebrating ones date of birth is of PAGAN origin.
“The various customs with which people today celebrate their birthdays have a long history. Their origins lie in the realm of magic and religion. The customs of offering congratulations, presenting gifts and celebrating—complete with lighted candles—in ancient times were meant to protect the birthday celebrant from the demons and to ensure his security for the coming year. . . . Down to the fourth century Christianity rejected the birthday celebration as a pagan custom.”—Schwäbische Zeitung (magazine supplement Zeit und Welt), April 3/4, 1981, p. 4.
“The Greeks believed that everyone had a protective spirit or daemon who attended his birth and watched over him in life. This spirit had a mystic relation with the god on whose birthday the individual was born. The Romans also subscribed to this idea. . . . This notion was carried down in human belief and is reflected in the guardian angel, the fairy godmother and the patron saint. . . . The custom of lighted candles on the cakes started with the Greeks. . . . Honey cakes round as the moon and lit with tapers were placed on the temple altars of [Artemis]. . . . Birthday candles, in folk belief, are endowed with special magic for granting wishes. . . . Lighted tapers and sacrificial fires have had a special mystic significance ever since man first set up altars to his gods. The birthday candles are thus an honor and tribute to the birthday child and bring good fortune. . . . Birthday greetings and wishes for happiness are an intrinsic part of this holiday. . . . Originally the idea was rooted in magic. . . . Birthday greetings have power for good or ill because one is closer to the spirit world on this day.”
—The Lore of Birthdays (New York, 1952), Ralph and Adelin Linton, pp. 8, 18-20.
#3. The Scriptures reveal at Psalm 51:5, Romans 3:23, 5:12, 19 and 1 Corinthians 15:22 that (1) ALL human descendants of Adam are born IMPERFECT (fall short of God's glory / grace).
#4. The world we are ALL born into is ruled NOT by God THE Almighty, but rather, by Satan the Devil (God's CHIEF enemy) whom 2 Corinthians 4:4 reveals to be "the god of this world".
The only begotten Son of God (Jesus Christ) was born into the world as a PERFECT / SINLESS human being (Heb 4:15; 1 Peter 2:22), however, absolutely NOWHERE IN SCRIPTURE will one find either Jesus Christ himself, Jesus' parents (Mary his mother and his adoptive father Joseph her husband) or any of Jesus' followers partaking in the custom of annually celebrating Christ's birth (in fact, the Holy Scriptures DO NOT even provide the date of Jesus' birth... But that's for another topic).
If none of the aforementioned partook in the custom of annually celebrating the Son of God's date of birth into the world and or their own birthdays... Why would anyone find fault in the fact that we who are Jehovah's Witnesses in this modern day *choose also not to participate in the custom of annually celebrating ones date of birth?*
Also, there are those who claim that Job 1:4 indicates that Job’s children annually celebrated their birthdays, however, Job 1:4 DOES NOT apply to birthdays.
Job 1:4 reads: “And [Job’s] sons went and held a banquet at the house of each one on his own day; and they sent and invited their three sisters to eat and drink with them.”
“Day” in Job 1:4 translates the Hebrew word yohm and refers to a period of time from sunrise to sunset. On the other hand, “birthday” is a compound of the two Hebrew words yohm (day) and hul·le´dheth. The distinction between “day” and one’s birthday may be noted in Genesis 40:20, where both expressions appear: “Now on the third day [yohm] it turned out to be Pharaoh’s birthday [literally, “the day (yohm) of the birth (hul·le´dheth) of Pharaoh”].”
At Job 1:4, hullédeth *does not appear*; only yowm is used in the Hebrew text. So it speaks of Job’s sons’ doing something “each one on his own day,” not ‘each one on his own birthday.’
So it is beyond certain that Job 1:4 DOES NOT refer to a birthday, as is unquestionably the case at Genesis 40:20. It would seem that Job’s seven sons simply held a family gathering (possibly a spring or harvest festival) and as the feasting made the week-long circuit, each son hosted the banquet in his own house “on his own day.”
As already stated elsewhere above in this answer, the Bible makes direct reference to only two birthday celebrations, those of Pharaoh of Egypt (18th century B.C.E.) and Herod Antipas (1st century C.E.). Again, these accounts were not depicted in the Bible in a favorable light. These birthdays were celebrated by people who clearly did not worship God and both accounts were marked with great feasting and granting of favors and both are remembered for executions: the hanging of Pharaoh’s chief baker in the first instance, the beheading of John the Baptizer in the latter. Everything that is in the Bible is there for a reason (2 Tim. 3:16, 17). Jehovah’s Witnesses take note that God’s Word most definitely reports unfavorably about birthday celebrations and so we choose to shun these.
It would also be good to consider that even if the account at Job 1:4 were talking about his son's birthdays, the Scripture DOES NOT mention Job attending. In fact, according to what is recorded at Job 1:5, Job prayed for them and was concerned that they had "sinned and have cursed God in their hearts." In addition, they all died on one of the occasions, so even if it was a birthday celebration, we would then have not two BUT THREE birthday accounts of which were marked by people dying. Hardly a stellar record.
Jehovah's Witnesses respect the fact that others who are not members of the unified worldwide Christian Congregation of Jehovah's Witnesses may and do freely choose to annually celebrate their date of birth and the date of birth of their family and friends and we do not interfere in such ones right to do so... You just won't find US participating (for reasons already stated) and we simply would like people to mutually respect our right and choice NOT to do so.
~ NOTE: I am a dedicated and baptized member of the unified worldwide Christian Congregation of Jehovah's Witnesses.