I speculate that some people are attention junkies, and performing on stage, any stage, is their fix.  Whether it is community theater, open mic nights, talent shows, or karaoke.  At least it is benign behaviour and doesn't harm anyone.

There are other reasons for these activities, and I don't intend to demean any of them.  They are all practice venues for aspiring performers, and hopefully are eventually outgrown for more professional activities.