Well, do you have a suspicion here? Why do you think they did/do it?
Beyond marketing, giving women a character to latch onto.
Usually it is simply to make the female character more appealing to the reader. Someone who is strong and capable... BUUUUUT because you are meant to insert yourself into our hero, those very same qualities that make her competent gets in the way of our reader being the hero to save her. So her abilities are subjugated.
As well usually the female character has more of a connection to the plot (there are obvious exceptions) because they want our male lead to be mostly an outsider so he learns things as we do. Thus often the female lead serves as the direct connection. Since our hero needs to "save her" he is usually the one who has to resolve her own plot.
But I think it just boils down to the creator just being uninterested in writing a good female character. Thus all he can muster is a good backstory and maybe a few good initial scenes and then he just basically drools over his ultra-bland self-insert main character.
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It is sad that typical Shonen shares a LOT in common with bad young adult books.
It is actually what helped me a LOT with understanding why animes I keep getting interested in end up being bad animes. The idea of setting up interesting plot ideas just to not follow through with them because you want to do something else (usually tepid romance) is quite universal.
So everytime I see an interesting plot synopsis in an anime I know why they won't follow through with it... because they don't care they are just trying to do something else entirely... usually boring action schlock.
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Ok Korean Liar game is quite different. They are more serious minded and WOW does Korean Nao lie a lot more... But so far so good.
I think I need a Korean to explain a few things to me since some of the changes feel like they exist because of cultural differences. For example the Liar game is more of a gameshow in this. Which isn't a bad idea, in fact I consider it a fair way to conceive the game, I just wonder if Korea has a history with gameshows that might help give this vision in a more cultural context.
The weirdest change, but once again a fair one. Is that to be in Liar game you apparently have to be a good person (though the host could have been lying).