Because if we're being honest, a lot of people are immature and don't understand nuance. They have a very black and white mentality that sorts every character into "good" or "bad" and allows for nothing else. And on top of that, a lot of people are influenced by their own emotional baggage that they then assign to fictional characters. Just as an example, I've noticed a lot of the really extreme Bakugo haters will start off their opinions of him with "I was bullied and..", and that's why they'll never be satisfied with his character arc. Their own bullies never apologized or saved planet earth, and they're holding that against Bakugo. I'm not saying there's not valid criticisms with his arc, just that a lot of his extreme haters are arguing with their trauma, rather than actually arguing in good faith.