I can see this from both sides. A couple of times a month I go to a singaround where people simply take turns in singing a song from where they are (you can stand up if the song demands it, but mostly people don't bother). It's a brilliant environment, the nearest thing to the supportive, immersive feeling of a tunes session you can get with songs. (If anything I find it more supportive than tunes sessions, but I'm generally more comfortable singing than playing.) And you can tell when people are more accustomed to singing from a stage to an audience - there's a certain kind of hi-how-are-you performance which can go down very badly at a singaround.

On the other hand, I'm also an occasional floor singer at a club with a stage/audience setup, and there's no denying that singing like that is an enormous buzz. And it does help if you bring along a bit of stagecraft as well as the ability to hit notes. So maybe the answer to the OP is "because it's fun". I wouldn't be without the singarounds, though.