Presumably to thwart SEO spammers or something. But is this seriously an issue? If someone is looking at someone’s user page presumably they are already of some interest and you’ll probably also want to follow any links there. So why do we need to copy-paste these URLs to visit them, instead of just clicking?
User profile spam is one of the most famous forms of link spam. That is, make a lot of fake user profiles with links pointing to your own sites. One advantage is that you aren't rubbing the spam in people's faces the way you would if you were posting fake comments on blogs.
Worried that crawlers won't find your fake profile pages? Well, just point some more fake links at your fake profile pages...
Worried that crawlers won't find your fake profile pages? Well, just point some more fake links at your fake profile pages...