Because there’s this great thing in life where people can choose what kind of person they wanna be. And honestly you seem like you’d like making memes and watching the Fresh Prince of Bel Air more than doing villain stuff.
He could be moving with the blow to rob most of it with hit’s power. Punching Superman while he’s staying stock still would be like punching a brick wall with all your might.
You’d likely shatter your hand and tear open your knuckles, at the very least.
This is just Superman getting the point across without destroying the guy’s hands.
Superboy 