It isn’t just a question, it’s straight up loaded with implications, most blatant one being to specifically stress ‘adults’ twice, as if it wasn’t a question to you if it were children.

People watch people playing sports, telling jokes or funny stories, acting drama, dancing… playing games fits that list so seamlessly that I can’t help but wonder wtf is wrong with someone asking why people would do it. It’s blatantly obvious loads of people like watching something they find cool and isn’t too mentally engaging, to wind down.