An interesting take I've seen about this is that at some point, a VTuber begins to view their own model as competition for their flesh selves. They want the kind of affection and (let's be real here) lust that the fans give, but at some point they think that it's not really "them" that the fans love, but their persona (probably not unjustified).
Then the brainworms start. With time their thought process may start running along these lines:
"Yeah, my fans love me, but only because I'm an anime girl who's curvy/stacked/got legs for days"
"My fanbase is growing, but I'm not sure if they will really stick around for 'me'."
"If they ever find out I'm just some average or homely girl, they won't love me anymore"
"I can't take these thoughts anymore, I might as well post and see how it goes. At least I know that the ones who stay are really here for me. Just once can't hurt."
Of course, each girl will have a different attitude toward this, but you see this happen more with indies, because there's no corpo rules that can make them second-guess fleshposting. And yeah, there are those unscrupulous e-girls who really just do it to farm simps (looking at you, Froot), but I imagine many other chuubas who fleshpost do it for the reasons above.
My take: It's probably inevitable for many of the kinds of girls who become VTubers, but I don't like them doing it on main. Tonya and Yuuna's approach of having a semi-open alt with fair warning is a better (if imperfect) approach, since people who don't care for it can just avoid it:
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Worth noting that this is different from a Mint and Kiara situation where they had to deal with a large influx of fans who just showed up one day on their RL accounts, not a designated alter ego for when they want to immersion break.
TL;DR: VTubers often want to be loved beyond the model. Understandable, but immersion breaking shouldn't be taken lightly.