As the abstraction of the act of streaming from its individual particularities, NPC streaming presents the distillation of the streaming profession, resulting in maximal efficiency in revenue generation. A quantitative increase in this kind of stream will eventually result in a qualitative transformation of the industry where it will be sublated by an entirely novel mode of production, namely the streamer-capitalist who relies entirely on the viewer for both the production and the consumption of their product, and who will eventually become a node in a never-ending tree of subcontractors.
I don’t think it is an “NPC fetish” but rather a fetish for ordering a woman around. They pay her money to act like this not because the NPC behaviour is what they like, but because they like the idea of her being mindless, like a robot, and just obeying their commands.
While I agree that probably the most famous ones are women there seems to be men too who are doing that, so I don’t know if you can blame everything on that.
Women and gay men have fetishes too though. But yeah, it probably isn’t just that.