Ok I think I know.
Writing long and well thought out RMT's takes up long amounts of time. A lot of people don't like spending time improving other people's team instead of their own.
Also, a lot of RMT's are new users that know little about this kinda thing. So they can just improve their teams by learning the basics first,so just a few comments and your off!
another thing is a user might not have any suggestions on a team, especially compared to the user actually asking the RMT. And for very small improvements a comment is all it takes. You don't have to explain things more than you should have too right? Many people think so.