Cause he was treated like an 8 year old piece of trash his entire life? You'd be the same if you were in his position. If you are told something enough, even if you deny that you believe it, your subconscious will still have the uncertainty of "what if they are right? Maybe I really am trash." And that isn't something you can just change on a whim either.
Besides that, He got beat up everyday, so his body still remembers that feeling of powerlessness, even if he gained a huge amount of strength.
@Fireshi dude clearly realised that his one punch can kill them, but still was trying to not hurt them. They tried to kill him.. I would've torn them apart even if I was treated worse than him. And there is no deep logic behind this they don't write stories on basis of IRL practical, they just make dûmb dense Mc who's OP who cares for everyone so that they can make him sympathetic towards bîtches who'll appear in future chapters to give mc a harem
