i'll answer that for you.......jealousy imho.
they see a pretty girl who is making an easy living 'ho-ing' it up a bit, dressed a touch provocatively. tight/low cut tops - but no more so than you'll see out round any town most sunny saturday afternoons - but those girls aren't making money or giving them blue****/gushy gussets while sat in their living room, so they are fine to 'put it out there'
These girls also upset them because
a) they are infinitely more attractive than them (visually)
and/or
b) wouldn't look sideways at them irl
so instead of just ignoring or flicking over to the underwater basket weaving twtich channel they should be looking for they decide to get all frothy and start crying like judgemental, pious, extremists accusing them of being glorified porn stars/animal abusers/harlots/ or whatever other sin they can level against them. while demanding their sad little voices should be heard and these evil entities be shunned by society and locked up/flogged/executed or whatever other punishment they feel they deserve.
i personally find a lot of the 'twitch girls' to be quite amusing (or rather the idea of them), most appear to have the personality of a chimp with learning difficulties with the game skill to match. happened to see a bit of a gamer girl 'danuca' earlier, now maybe she was having an off day but she was utterly gash at pubg - and that's saying something coming from me - but she had in excess of 1200 people watching her play. she had a sore throat so was coughing and spluttering, barely talking and when she did talk her voice and accent would have made your ears bleed - still had 1200 folk watching her and subbing and donating, now that's some good marketing skills. shroud et al who are godlike with their game play i get, others who's names allude me but are less godlike tend to be entertaining, funny even. this girl, however, had apparently zero going for her other than being reasonably pretty. bonkers world we live in but it's no skin off anybodies nose at the end of the day.
you're quite right, the frothers should be more upset that twitch isn't applying it's rules fairly. so why aren't they taking their anger out on twitch instead......well, simply, because they ain't jealous of twitch