At least drunk karaoke singers exhibit some personality, and play to the crowd a little bit. Most of these folks are just singing. Singing well, yes, but that's all it is.
I like watching people sing well, but more than that, I like watching people having a great time. That's the problem with these shows. The stakes are so high, and they feel like there's so much riding on every performance, that they don't seem to let loose and have fun.