I don't know about everyone else, but have you ever been unable to actually spend money on games you wanted to play, because of not having the capital for it? Or perhaps, having only access to one type of console but not being able to play the others? And it's not like emulation is always possible, since the more advanced the hardware, the more complicated that becomes.
Of course, I'm not saying all streamers are amazing and what not, but sometimes, people can't play games they want to, and watching someone play for them is useful for them to know what a game is like. In other cases, maybe it's just the parasocial relationships thing where nerds can't have friends so they'll pay someone online to be nice to them.