I'd argue that's not completely true because in that case why not just watch porn or why watch non-sexy video game streamers who also get plenty of viewers?

I do think you are partially correct though in a way that makes it a little more depressing, I think a lot of viewers see a cute girl playing video games specifically as a combination they like so they can fantasize about specifically having a "gamer girlfriend". But if they like that, then they're also there for the gameplay or will inevitably get at least a little invested in it as they watch.