Matthew 28 says he appeared to the two Marys, then to the 11 disciples in Galilee.
Mark 16 says he appeared to Mary Magdalene, then to 2 of the disciples, then to all 11.
Luke 24 says he appeared to 2 disciples on the road to Emmaus, then to the 11.
John 20-21 says he appeared to Mary Magdalene, then to an unspecified number of disciples (specifically not including Thomas, maybe all 10 of the others, maybe not), then to the 11 indoors, then to 6 disciples on a beach.
Ephesians 4 says he "descended into the lower parts of the Earth", which the Apostles Creed interprets as "descended into Hell".
1 Cor 15 says he was seen by Peter, then "the twelve", then "over 500 of the brethren at once", then by James, then by "all the apostles".
Acts 1 says it was 40 days from the resurrection until his ascension.
There may be other references to post-resurrection appearances that I'm forgetting.
None of these claims to be a minute-by-minute of Jesus's activities after the resurrection, so it's not surprising that some mention incidents that others don't. Mark's mention of him meeting two disciples and Luke's mention of meeting 2 disciples on the road to Emmaus are probably the same incident, but it's tough to prove. Apparently he met various groups of the disciples at different times, so figuring out which meetings in different accounts are describing the same event is not necessarily obvious.
As to why: Jesus gives (at least) 2 reasons: John 14:1 "I go to prepare a place for you." Normally understood to mean Heaven or the New Jerusalem. And John 16:5, "It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you."
Why couldn't Jesus stay here and bring the Holy Spirit, or send the Holy Spirit and then come back? I have no idea, but the plain reading appears to be that Jesus said this would not be possible.