Not all of them are women. Plenty of dudes with good vocal ranges. And "teenagers" is hella optimistic given some of the most popular designs.
So the vtuber shit started in Japan where because of So Ronery (and Idol Culture; Idol is basically a Nippon Popstar) people would hang out in 'personality' chat rooms and talk. People who didn't want to show face (because so shamfur) used disguises (since mainly of them created fictional 'streamer personas') and avatars. Eventually mocap technology let them map a pretend face, and eventually limbs. There's some further gay shit about "virtual idols" but I'm not doing a full Slant Psychology deep dive.
tl;dr Origins are weeb shit. When shit seems bizzarre, just remember this started with the Nips.
It started gaining traction in the US with "Projekt Melody". Basically past-her-sell-by-date Camwhore mapped to a 3D anime, doing camwhore shit and virgins who have never touched a real woman ate it up. The concept was proven, expanded, and entered the "Its ok, she looks 11 but the woman doing the voice is like 35 so its fine". Its the same cringe e-thot simp behavior you normally see, but now with 3DPD the weebcels are fully behind it, so its this huge vortex of autismal simps. I mean you've seen the cringy shit people do for/with 'waifus'. Now imagine if that Waifu was actually interactive.
tl;dr After Motteing with a purplehaired big-tiddie anime whore, they then quickly Bailey'd with pedobait. These pedophiles discovered this one 'weird' to indulging their sick cravings without getting arrested, Eppstein hates them.
The troons & tankies are against it because it is an alternative to going jailhouse gay for incel nerds. The
girls women people humans individuals doing this shit are cartering to social anxiety millenial shut ins, so they can pretend they have friends. Since the couple of times they went to a party they just hung on the outskirts listening to other people talk anyway. Anything liked by theorhertically straight males is wrong.
SA Mods are against it because it means less chance of them getting access to proxy Child Porn.
tl;dr People who aren't troons are getting attention and money, and that is very wrong.
Banning vtuber fans, much like banning furries, is the good and right choice. The only issue is that the same people complaining about creepy 3d anime pedobait voiced by adults, is that they proceed to go and pound their prostates to also underaged cartoon characters because of "queerness".