Interesting, you need to check your sources.
I think you're missing the point.
Yes some 90% of Mark is found in Matthew, but is all of that directly quoted.
No, Matthew's adapted it to his own situation and requirement.
Some of Jesus' says are not "word for word". look at the parable of the Sower in both texts.
And yet a misquote of Isaiah is exactly the same in all three, which is so highly unlikely, it can only be that two copied from the same source without double-checking the reference.
Matthew should have double quoted Mark if he copied from Mark ...
Not if he's using Mark as his source, of course not.
The point remains, if Matthew was an eye-witness, why is he so dependent on someone who was not? It's clear Matthew actually had a copy of Mark in front of him, and in some cases copied it word for word. Same with Luke. Common-sense says Matthew was not the disciple – not the Matthew of the Twelve, at least. He might have been a follower.
It may be that Matthew had the Hebrew materials Papias mentions (which are lost), a sayings or discourse document, and wove this into Mark's story. Neither Matthew nor Luke challenge Mark's chronology, and yet John's is different, and more likely the more accurate. If the scribe of 'John' was not the disciple, I believe him to be a disciple of the disciple. as Papias and Polycarp knew John, and neither mention a scribe, it's more likely that John's Gospel is a first-hand account.
You are welcome to believe what the tradition asserts – the Church is more circumspect – I believe Matthew might well have had the lost 'Hebrew Gospel' and wove that onto Mark.
I believe Mark writes Peter's testimony, but the Church doesn't insist on that, again it's Papias' telling.
I believe Luke spoke to whoever he could, including Mary the mother of Jesus
And I believe John was the disciple.
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Again, all my beliefs. Strong – but not inconclusive – evidence to allow I'm right. Not enough evidence to tell me I'm wrong.