Because they’ve made their identity the bad behavior. You can’t change behavior when someone has adopted it as their identity. Any criticism is perceived as a personal attack. Thus, the only approach is to point out that being an asshole as your identity is toxic and to be avoided.

Everything you just said is only reinforcing my belief that it's best to separate the behavior from the identity at all times, but especially when criticizing it, so that we don't spread the bad idea of linking bad behavior and identity in the first place.

That's just a return to the status-quo, which led to where we are. Giving advice of "don't be an asshole" didn't work because people made it their identity. Switching the advice to "don't make being an asshole part of your identity" is at least an attempt to change tack and address the weakness of only targeting behavior.

I can respect teaching people not to make it their identity, but the term "toxic masculinity" does nothing to teach people, it just bundles the toxicity to their identity.