>>268334504>>268335420Two main reasons.
Most people who "beg" don't actually want to find one, they want strongest weakest timid nosex humblebrag to complain about, complaining is a drug more addictive than crack. When they run into something that meets all their criteria they ask for, their brain immediately comes up with some bitch made excuse to drop it, from "I don't like art", to "too much sex", "mc is too proactive" etc. etc. all of these actually translate into simple "there's no drug in it for me". We may beg for it here, but no one is looking for those, in reality.
Second reason why they aren't as common is mostly publisher censorship, look at something like Lonely Attack manga vs WN for further information. If you fill your series with adult content you're genuinely severely hurting your adaptation chances, sex is treated by publishers as a more taboo thing than gore and torture porn as long as naughty bits covered up.