I think it has to do with the kinds of stories these characters are used to tell. Batman is a tortured billionaire who tries to use his vast resources to solve the problem of crime single-handedly, and he keeps people at arm's length because he's afraid that personal ties will endanger the mission he's given himself (or something like that, Batman scholars feel free to chime in if I got it wrong.). Spiderman is a story about a broke kid trying to make a difference in the world with the limited resources he has. Similar goals for both characters, but different preconditions make the stories meaningfully different.

I think these flaws are what endear fans to a particular character because they struggle with the same problems (overly self-reliant, broke as hell) and if you have a character grow past them, you're now telling a meaningfully different story. Might still be an interesting story, but I get why people who love these characters would consider some changes to be dealbreakers.

This is kind of a foundational feature of serialized character stories: if you want to keep telling stories about the same characters over and over again, they can't fundamentally learn or grow or change meaningfully, not permanently anyway, because then the appeal of the character fundamentally changes, so you get characters like Batman who are stuck in this sitcom-y eternal purgatory of constantly slamming their heads against their own limitations, and still failing to grasp the root issue. And really I think, it's not for them to figure out. Their stories are there so that we can see our own flaws in them, and learn from them. And once we have, Batman will still be out there, being a lonely nerd for other lonely nerds to identify with.