Well he does in the Greek. I think the "rough edges" are often smoothed out in the translation. That's another argument, by the way, for Markan priority. The idea is that Mark wrote the original and the other gospels smoothed out the primitivity of the language by adding sophistication. I don't mean any of that as a pejorative. I think Mark is a piece of really good literature. However, Mark seems to be closer to the oral tradition as you say and is therefore most likely to be the earliest.
The Sermon on the Plain is an edited version of the Sermon on the Mount. Had he been copying, Mark could have included an edited version. Ghandi (a non Christian) regarded the Sermon on the Mount as the greatest piece of religious literature ever created. It seems odd that Mark would omit it entirely.
Furthermore, if the Sermon on the Mount really did start as a literal sermon on a mount, it would work pretty well as drama. After all, it started out as oratory.
And you also can't explain the omission of the Lord's Prayer or a nativity or any post resurrection stories.
Mark's frequent use of "and" and "immediately" seems to be due not to primitivity, but used deliberately to increase the pace of the narrative.
Wikipedia says that scholars view the omission of the SotM from Mark is evidence for Markan priority.
To paraphrase the book I linked to earlier in this thread, "This argument has been repeated by scholar after scholar, and indicates the assumption that if Mark had known about the Sermon on the Mount, he would have included it."
Three points are evident (taken from the same book):
First, this argument is based on the assumption that if Mark had known about something, he would have included it. This in turn is based on the scholar's inability to think, 19 centuries later, of a reason why he would not have included it .
Second, this line of argument leads to the conclusion that the author of John's gospel was also unaware of the SotM and the other parables and teaching of Jesus, since he doesn't include them either.
Third, this argument assumes that Mark didn't have knowledge of other teaching material such as the SotM; however, we know from Mk 4:3,33, 12:1 that he
was aware of other such material.
Therefore, he could also have been aware of the SotM. The statement in Mark 1:22 could indicate this, since it is almost word for word the same as Matthew 7:28-29.
Matthew 7:28-29 refers directly to the SotM, and the peoples' astonishment at Jesus' authority as a true Rabbi as contrasted with those others who used that title. Perhaps Ghandi's response to it is similar to the astonishment of the crowds, and evidence of the originality of Matthew's statement concerning that astonishment.
If Matthew was first, you are claiming that either Mark or Luke made up that story.
Or simply used another source.