Actually they were written at 10-15 years intervals as snapshots of the current teaching in the Jewish Christian churches, as they learned to reproduce the sermon on the road to Emmaus in their studies of the OT 'mystery hidden from the beginning'.
Mark was the earliest and the least proficient: He started with the preaching of John the Baptist.
By the time of Matthew, they discovered that Israel was a shadow of Christ, and he begins with Abraham.
When Luke wrote, they had discovered the guys before Abraham were also shadows of Christ, and he begins with Adam.
John learned to read the formation of words, where words get their meaning from the combined meaning of the letters. He begins by exegeting John 1:1-4 from the first three words of Genesis 1:1.
They delivered each gospel to the Greek churches by messengers who used them as the outlines for sermons referencing the OT.
The Greeks did not wish to learn Hebrew, and did not understand references to the OT nor the mystery. They added elements from Greek philosophy and myth not found in the OT to their doctrine and enforced it by subjugating the laity.