You make an interesting point. Have you ever wondered why our Lord did not instruct His Apostles to document events in writing. Had He intended that these be the foundation of faith for future generations, it seems He would have written them down Himself. Instead, He sent His disciples out to preach.
When we think of convincing others, doubting Thomas comes to mind - "Unless I see the mark of the nails in his hands and put my finger into the nailmarks and put my hand into his side, I will not believe." How weak we are! We need portents and signs to believe. St. John recognizes human weakness and writes:
Now Jesus did many other signs in the presence of (his) disciples that are not written in this book.But these are written that you may (come to) believe that Jesus is the Messiah, the Son of God, and that through this belief you may have life in his name.
John 20:30-31
Matthew is thought to be an eyewitness, although the book of that name was apparently written and translated from his account and not by Matthew. Mark and Luke were possibly also written from the same Matthew document and appear to be collections of folksy daily doings. John was also an eyewitness and wrote his book directly in passable Greek and in a different and systematic manner. But, we have what we have in writing and the essentials are there. The eyewitnesses are long gone, but that shouldn’t matter.