Why is much of Jesus life not discussed in the Bible?

There was really no point.   In His (natural) life as a baby, child, young man and adult, until He was manifested to be Baptized by John the Baptist as The Lamb of God who takes away the sin of the world at the age of 30, were irrelevant to us. (and consequently, we are only told the one relevant thing that occurred when He was 12)

Once Jesus was in this world, anyone who wanted to receive His Christian message - the gospel - could be saved, but those years were only three and a half - then Jesus was crucified and as we all know, resurrected and ascended back to heaven in His glorified body to sit with the Father.

So the three and a half years were the only relevant ones for us and now of course, due to the fact that He died for us and therefore, He is our Saviour and we live and move and have our being in Him.

John 21:25  There are also, in fact, many other things that Jesus did, which if ever they were written in full detail, I suppose the world itself could not contain the scrolls written.

John’s point was that much more could have been written, but there is enough in the Bible to prove beyond a doubt that Jesus Christ is the Son of God.

Jesus was baptized at the age of 30 and that is when his mission began. Upon his return from the desert, Jesus chose 12 disciples who became the Apostles of Christ. Most of what we know about the life of Jesus was written by the Apostles.

He was a carpenter and died at age 33, his mother was Mary and he healed people and died for our sins, he got baptized, he ran off from his parents to preach about God, he fasted had no food. That's quite a lot

Bibles include Gospel of John, the teaching of Christ and the work of his some disciples and other religious folks. Those other people and some unsuccessful disciples of Jesus ruined the teaching of saint John by editing the text and adding their beliefs. 

Sorry please don't mind, due to privacy issues I am not revealing my identity. I don't want to be admired by anyone. You need proof I can give you proof. 

1. In the books of Moses it is written earth is flat and there are 4 corners of it. 

2. In Christian’s bible, where there is Gospel of Saint John, there Christian leaders also added their beliefs, new prayer, and other wrong teachings. Know when Christian religion was made, at that time true saints were not there with Christians. Bibles are not checked by any true Saint. You need to remove all text of Christian from Bible. 

Because he appeared when he was 30

Most of the less-than-perfect aspects needed to be dropped once the fiction was added that he was God.

-𝒞ℴ𝓇𝓋𝓊𝓈 ℬ𝓁𝒶𝒸𝓀𝓉𝒽ℴ𝓇𝓃ℯ

Because in about 80 C.E the first gospel writer Mark (not his real name) didn't mention it. The following writers Matthew and Luke (not their real names either) mostly just copied Mark and seem to lack the imagination to fill in the gaps.

The motivation for the gospels is cult based, not to tell the life story of a fictional person called Jesus.

Because it doesn't matter, What Matters is what happened after John Baptized Him

Because Jesus' teachings, his ministry, are what's salient, not trivia about his life. The gospels are called "gospels" instead of "biographies" because they recount Jesus' gospel, not Jesus' biography. By the way, this isn't unique. The Torah, for example, skips over much of Moses' life, Moses going from baby found in the Nile in one verse to full adult in the next.