An interview of Craig Blomberg, conservative Christian New Testament scholar, by Lee Strobel, in his book The Case for Christ, p. 28:
Strobel: Why would Matthew, purported to be an eyewitness of Jesus, incorporate part of a gospel written by Mark, whom everyone agrees was not an eyewitness? If Matthew’s gospel were really written by an eyewitness, you would think he would have relied on his own observations.
Bloomberg: It only makes sense if Mark was indeed basing his account on the recollections of the eyewitness Peter. As you’ve said yourself, Peter was among the inner circle of Jesus and was privy to seeing and hearing things that other disciples didn’t. So it would make sense for Matthew, even though he was an eyewitness, to rely on Peter’s version of events as transmitted through Mark.
Strobel: Yes, I thought to myself, that did make some sense.
Gary: Good grief. They have a harmonization for everything!