why is it that some fighters have an obvious flaw/bad habit that isn't fixed?

I'm kind of curious as to why, don't the trainers pick up on this and try to fix it? What's the reason for this? Is it the fact that they've been doing it for so long that they can't change etc? or is it just up to the person to fix it and they just never seem to do so.

some guys lean in too much, stance is akward, punching technique and all that.

pacquiao tends to overreach like crazy, i'm not hating i do the same thing and can't help it but it's getting better.
Cotto leaned in too much on the left back then, did some weird squat to slip punches etc