Copying over possibly tens of thousands of posts all in one go for every single account transfer can be quite demanding on a server. The only platform (AFAIK) to try this was Firefish, but I think it got slowdowns when they did this?
Also, bear in mind copying posts over to a new server will break all the threads. The replies, the links, the boosts etc will disappear. If you leave the posts on the old server with a redirect to your new account, this keeps everything intact.