Weirdly enough I usually keep shit like youtube on in the background and Ive got DeadwingDork on and I came back into the room and hes playing some videos of like women having trouble starting cars and you KNOW its fetish shit. I dont have timestamps but you can click around the video.
I think a lot of it is like clips4sale type shit, either pirated from there or just an ad for some porn channel there--half of it looks like its shot in the late 90s on some shitty camcorder by some Serbian and the girls gonna wind up hanging from a meat hook in some warehouse in Silent Hill, the other half of it is obviously young girls who are being paid to act and do this and theyre bored and confused why theyre being paid so much to do it--but they know its porn and they dont even have to get naked.
I think its a lot of factors but I think a large part is that all this is automated mostly. Youtube is such a huge international platform theres probably a hierarchy of shit that will trip flags, but these channels are so small and its so niche its not on their radar. Something like the post you quoted where the video is titled "HENTAI Forest Story download link", you have to be searching "hentai" anyway, and it is a censored looking 5 second video with presumably only a download link. Its so niche that its not going to get as many flags as bigger channels like "Oh shit a bunch of pearl clutchers are mass flagging this guy for teaching his dog to raise its right paw in a Nazi salute".
You could try flagging terms that arent necessarily sexual but usually are, like if it was a video about "slut shaming" obviously youtube isnt going to step in that quagmire and slut shame Laci Green and strike her channel, and even if you flagged those terms to be TV-MA or some shit, kids lie about their age on the internet all the time so it wont change much.
I also think that theres a general desensitization to sex shit in a certain regard, and especially on the internet--instagram can be a thirst trap for example--so when youre watching "Ariana Grande concert compilation" or whatever and in the comments its normal but then theres like one or two horny Indian comments like "wow so hot bb @3:24 bobs and vagene I COOMM" you kinda roll your eyes and laugh; the average person doesnt know how far that shit goes. I know toy channels were dragged for being weird a while back before they started manually looking into Elsagate shit, but I could buy it that it was genuinely like some Colombian family who thinks they really just stumbled across some weird American obsession with slime for their kids, but really its just the average parent looking at parenting or toy review vlogs are watching one or two videos and moving on but then you get one guy whose fetish is for people getting trapped in slime and being forced to play with transformers watching every single one and sharing them through playlists and offsites to other people like that.
