I think the take-off of VTubers as the tech got much cheaper is for a few reasons:
Anonymity: they can still have normal day-to-day lives without potentially getting mobbed by fans and less likely to get stalked.
Fantasy Escapism: you can look like whatever you want.
Convenience: even the prettier Twitch girls probably get tired of putting on make-up every time they go to work so an avatar means they can skip that and hop online whenever they feel like it. I'm sure that's a factor in some of the male ones as well.
I think the take-off of VTubers as the tech got much cheaper is for a few reasons: