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Because think about it...
Kirishima has some pretty bad quirk envy. Everyone else's quirk, in his own eyes, is better than his. He has trouble seeing his own self worth, nevermind his quirk's worth.
And then he trains his ass off and unlocks Unbreakable. The ultimate shield move. He can block Bakugo's blasts, tank hits from fucking Rappa and Ojiro and whoever else is throwing massive blows his way. And there's his worth, right? There's what he's made for. It isn't an assigned roll, no one ever told him he had to be a shield, he figured it out himself.
The thing about self worth issues is that you only see the bad. So he never sees his hits, only his misses. He doesn't recall standing back up in that Rappa fight, just that he fell in the first place. He doesn't remember using the middle school incident as a turning point in his life to become the best hero that he could be, he only sees himself as a scared preteen, watching Mina stand up for people and that he was to weak to do it himself. He doesn't see the fact that he saved Mina and got the tranquilizer into Gigantomachia's mouth, just that it didn't work immediately and he thinks that he failed.
So he works harder. Trains more. Pushes himself to the edge and then even over it; creates a new line in the sand every month on how hardened he can become. How fierce a hit is that he can take and keep moving. Because he wasn't strong enough in middle school. He wasn't strong enough when Ms Midnight died. And he wasn't strong enough again during the war and Mina exhausted herself and her quirk.
So, in his mind, his option is to become the best damn shield anyone could ask for. By shielding those around him, by becoming so damn sturdy he can bench press a building, he's worth keeping around.