We need to realize that the Gospels are NOT modern biography, but theological writings showing how Jesus was God's NEXT ACT in the history of the redemption of Israel and the Gentiles. So the selection criteria is completely different because the QUESTIONS the intended audience raised were also different than modern biographies.
Nativity stories are important because the it's critical for this God's next act that Jesus was born from the right family (descendant of David), from the right town, that he is fully man and fully God (compare with Greek myth of half God heroes), that his birth fulfilled prophecies, etc. Why does fulfilling prophecies matter? Prophecy by definition is a message from God. So if God promised something in a prophecy (which he DID a lot in Isaiah, Jeremiah, Ezekiel, Daniel), his character DEMANDS it to be delivered), and it's incumbent on the gospel writers to SHOW IT TO US !
Jesus's suffering and the meaning of his crucifixion and resurrection is also critical for this purpose, as well as his role in the new covenant as the new high priest (mediator), the second Adam (first fruit God's raising of the believers), and so much more.
Imagine having to pack all of them into a manageable scroll / papyrus where there was no computer / copy machines. It's important to be brief, so each sentence is packed with meaning, and the literary quality of its composition is amazingly dense and full of deeper meaning and symbolism. Compared to this dense arrangement, modern biography is too redundant and wordy.
Only later fanciful and curious people who need filler to satisfy their imagination started to come up with the apocryphal books, but those were NOT part of the apostolic tradition. They are more like historical novels, or worse, a device for promoting heretical non-apostolic teaching about Jesus (like the Gnostics). But going back to the original purpose for the Gospel, probably Jesus's experience in his teenage years are not relevant to convey the fulfillment of the prophecy and about God's next act in redemptive history.
Of course I wish that the Gospels were 10 times as long to fill in the details. Oh, how I wished there were Internet and stack exchange Q&A forum like this to record Q&A with the apostles or even Jesus himself for posterity ! But we'll have to be satisfied with what we have and make the best out of it with the help of responsible Bible, literary, and history scholars.
Hope that answers your question.