Traditional gender roles are much more ingrained in Asian societies. Although the internet and globalization certainly brings in modern western perspectives for the younger generations, traditional thinking and cultural norms won't be so easily changed. There is an underlying misogynistic view on gender.
Porn in the west has the underlying vibe of female empowerment and sexual liberation. You don't see that much outright rape or non-con stuff. You see much more porn where the females are on equal footing or even leading the interactions. On the other hand, a much bigger percentage of JAVs show subservient females that are corrupted by some dude. Sex itself is portrayed as a form of male dominance. That's why the default JAV acting is portraying the helpless woman squealing and shuddering as she succumbs to pleasure despite being "pure."
Just taking one specific example and how the west and east differs in their outlook, let's look at NTR. Western NTR, aka cuckold fetish, usually focuses on the humiliation of the male cuck. The females are empowered, and even borders on femdom. Of course, there will be the hiarchy of the bull at the top, but the female is not a helpless victim of corruption here, but rather an active participant. On the flip side, Japanese NTR is most often portrays the in depth corruption process of a pure girl who becomes a plaything of some big dick baddie because she is "just a woman."
That's why the vast majority of Japanese female protagonist games are corruption rape games. Females are expected to be subservient, obedient and meek. Anytime you see a strong female, that is pretty much just a set up to see her fall and brought down to the level "where she belongs."
Now when you ask why female protag rather than male, I'd just make a guess that Japanese people self insert less, but I have no data to support that. The main appeal and lewd aspect is seeing how these females fall, rather than living out the fantasy of some male chad. It's just porn.
Of course, you can't generalize the whole industry. There are many female protag corruption games in the west and male protag feel good harems from Japan (Isekai is pretty much a quintessential male harem fantasy, and it's super popular, especially among youngsters).