Why Doesn't Matthew Say More About His Conversion?

In another thread about Matthew's authorship of the gospel attributed to him, a commenter wrote:

Hi Jason, the first person I heard use Mt 9:9-13 to challenge Matthean authorship of The Gospel According to Matthew was Richard Bauckham. It has long troubled me. The external evidence highly favors the view that the apostle Matthew wrote the gospel attributed to him, but it seems so counterintuitive that Matthew would use Mark’s account of his own conversion rather than write his own. That is the one place you would expect him to pen his own unique account. You rightly point out that Matthew did not copy Mark verbatim, but the differences are so miniscule (no greater than in most other places where Matthew uses Mark) as to be relatively insignificant. Minor changes to Mark’s account of Matthew’s own conversion is not what we would expect at all. It’s the most personal element of his story. He adds lots of non-Markan material throughout the rest of His gospel on matters he may not have even witnessed Himself, so why use Mark’s version of his own conversion story rather than telling his own story in his own way with a lot more detail? It doesn’t seem probable and counts against Matthean authorship in my mind. How do you reconcile this?

You can find a collection of some of my material on Matthean authorship linked in a post here. Steve Hays has written some posts on the subject as well, like here. Here's my response to the commenter quoted above:

See my earlier article I linked above, the one here. If Matthew delegated the composition of his gospel to one or more other individuals, then it’s not a matter of what Matthew wrote. It's a matter of what others wrote on his behalf, with his approval of the final product. That was a common practice in the ancient world, and it's common today (ghost writers, group authorship, editorial assistants, research assistants, etc.).

However, even if Matthew hadn't operated that way, where did Mark's account of Matthew's conversion come from? If it came from Matthew (with or without intermediaries), which is a reasonable scenario, then why think the account found in Matthew's gospel should differ more from the account found in the gospel of Mark?

You suggest that Matthew should have included "a lot more detail" about his conversion. Why? He wasn't writing in a modern American context, in which individualism, writing lengthy accounts of your experiences, and such were as popular as they are in our culture. Even the vast majority of liberal scholars think that Paul wrote several of the letters attributed to him, yet Paul didn't provide "a lot more detail" about his conversion in those letters, including the letters where his conversion is mentioned. And Paul's conversion was in some ways more significant than Matthew's. If Matthew was asked to compose a gospel in response to the popularity of Mark's gospel, which I think is most likely what happened, then the people who approached Matthew about writing a gospel probably would have been people who already knew a lot about him, including his conversion. Besides, he was writing a biography of Jesus, not a biography of Matthew.