I have learned that about 55% of Matthew’s gospel is just Mark’s material. If Matthew was an eyewitness why would he use 55% of Mark’s gospel? He even uses material from his gospel for events that he witnessed such as the last supper. In the last supper, the last part in Matthew and Mark is identical, they both say “When they had sung a hymn, they went out to the Mount of Olives.”
A good question. I believe that all three of the synoptic gospel writers (Matthew, Mark and Luke) relied to a significant extent on the oral tradition which had been used in the churches up to the 50s AD. I believe that Matthew’s similarity to Mark is more due to the fact that both relied on the oral telling of the gospel that had been in place for 20+ years, than his reliance on the gospel of Mark itself. Yes, Matthew was an eyewitness. I believe that Mark was an eye-witness to at least some of the gospel events as well (he is probably the one who lost his robes when all fled in Mark 15:51-52)). However, the synaptic authors stayed fairly close to the oral traditions which had been in use in the church from the earliest times. This explains, both the similarities, and the differences, as there were probably some small local differences in the oral traditions, as well as differences in the experiences of Matthew and of Peter/Mark. Neither Matthew nor Mark at this point, about 30 years after the events, could remember word-for-word the teachings of Jesus, so they relied in part on their own memories, but also on the oral traditions. That is my thought on this question.
By the way, you are assuming that Mark was written before Matthew. Scholars still debate this, but it is approaching consensus status that Mark wrote first. If Matthew published first, that would change the equation for sure. Again, we should not discount the influence of the orally transmitted gospel accounts which preceded any written gospel.