I think a large part of it is the parasocial aspect. People get to know streamers (or at least, the streamer’s persona) and form attachments to them over time. The same way someone wants to see how their favorite character’s arc plays out in a show, people want to watch streamers because they feel a personal connection to them.
Even though that connection is almost certainly one-sided, (except maybe for smaller streamers who actually interact with all of their viewers on a regular basis,) it doesn’t stop the viewers from forming those emotional connections. And that somehow makes them more “real” than some random piece of ass on pornhub.
I think a lot of it stems from loneliness, and wanting a personal connection. Pornhub is great for porn, but it’s undeniably commercialized porn. Lots of people will likely end up viewing nude twitch streams the same way they’d view nudes from a significant other. Even if the quality isn’t as good as pornhub, it may be considered more desirable simply because the viewers feel like they know the person.