That's a bigger question, like why do people watch streamers in the first place. It's not for me personally but it's a huge worldwide multibillion industry so plenty of people enjoy it and / or aspire to be it.
Occasionally if they are genuinely funny or are a personality I care about, I find the streamer can be additive to the experience.
For example it was quite hilarious and interesting seeing Justin Wong of street fighter fame trying out Elden Ring. Same for Demetrious Johnson from UFC
But I can't stand streaming because the entire culture is infantile and lowest common denominator. It's the same problem I have with Short form video content and microblogging, that in order to optimally attract the large market that prefers that content, you have to do things that are extremely off putting to people with my interests.
So for example, anyone playing a game on twitch will be pushed, by chat and by the invisible hand of the attention market, to optimize for stupid things. Instead of a roguelike build that is good, you will make a roguelike build that is "memey" because that gets you a larger audience. There's an aura of forcing chaos and chat sycophancy. It encourages the kind of antisocial behavior that sucks, like people in twitch plays pokemon who spent the entire time trying to ruin it.
There's also the problem that half the platform seems to be just softcore porn. Or gambling now.
Many streamers also run a popular youtube channel, and you can recognize a massive difference between content made directly for long form youtube videos vs their streamed content. IMO the quality itself is drastically different.
Streamers who don't do those kinds of things remain niche.
"Normal" streamers are watched for a variety of reasons: maybe their personality is entertaining and charismatic, maybe they're really good at the game(s) they play (speedrunners in particular fall into this camp), and so on. There will always be viewers watching purely for parasocial reasons, but it's usually not the primary reason to watch them.
With vtubers, though, the parasocial element is essentially the full extent of the appeal. They're not very charismatic (vtuber humor is infamous for consisting mostly of loud childish noises and sex jokes), they're usually not very good at games and rarely dedicate themselves to one game or one genre. They are watched because the viewers are lonely and want to feel like they have a (girl)friend, that's about it.