Why do you act like male dragons do not do the same thing?
Also it is fans's fault vecause they suddenly start to hate dragons' dragon form, completely ignoring their existence.
Nah, I'm not particularly ignoring it, but I can only speak for myself and from my own experience. Being a man, I have read astronomically more shounen and seinen than shoujo or josei, so I wouldn't know exactly how common anthropomorphic dragons are in those genres, precisely as potential love interests (or simply for fanservice), though from my limited experience, I do know they are there. Of course in shounen/seinen, it's not uncommon to have male dragons turn into a humanoid form, but they are typically either combat rivals, old sages, or such. Now, I'm not saying in old-school fighting shounen rival male characters wouldn't be important in connecting with the readers emotionally, but let's be honest, new generation writers haven't really bothered with this as much, compared to the old famous titles.
Readers read what's available. Japanese fantasy genre has been overun by populist writers, especially in the isekai genre. There are probably a hundred webnovel chapters of isekai, or practically isekai fantasy, released daily, followed by a dozen chapters of light novels. New fantasy manga is then adopted mainly from these. The vast majority of authors have no talent to speak of, but the business models doesn't expect any. This whole scheme is best demonstrated by the phenomenon of when an author happens to come up with a novel idea, within a year there are ten other series that copy-pasted it as is, with zero shame.