I think on average female characters are made more interesting than male MCs to 1) attract popularity 2) allow for some sort of self-insert or wish fulfillment.

I do feel that the anime industry is sexist in the way it writes it's characters, which is the underlying reason why the female characters seem more interesting than male ones. Female characters are often slaves to traditional gender roles or cast in the complete opposite temperament, overcompensating to show that they are specifically not feminine, which in and of itself is just lending support to the existence of gender roles.

I totally get why it happens, but when you look at it how many anime have serious female characters that are not shown to be weaker than male characters or used for sex appeal? Not very many. In general women are shown to be vulnerable, feminine entities that fall in love with the MC because he's so amazing. Hence they are made as appealing as possible compared to the bland male characters that no one cares about.

It's always slightly left a bad taste of the industry imo, but I very rarely see anyone bring it up. Some anime obviously write great female characters that don't do that. Some parody or deconstruct the idea of the main female characters serving only as sexual objects (Trigger with KLK). Some unapologetically cater to audiences that don't care. But I would say the vast majority of anime contain subconscious sexist ideas that, while may not be obvious or even really matter that much, are nonetheless there and reinforcing harmful stereotypes and gender roles.

All of this is ultimately explained by the fact that the main audience for anime is men. And that audience doesn't really care about how interesting male characters are if there's a cute, feminine or beautiful, aggressive girl in love with the MC.