So the first thing you'll want to do in your career as a superhero is try to lift something huge. Hell, that's all they did in the last Superman movie and—oh no! That bus is falling off a cliff! Better go catch it!
You jump up and catch the underside on your hands. Great job! But, instead of the relieved cries of little Johnny and Mary as they're saved from falling to fiery doom, you hear screams of agony as your hands rip through the undercarriage, up through the aisle, your arms and torso now bloodied like some B-grade zombie. It's not your blood; you just impaled little Johnny from crotch to sternum.
You can thank the laws of physics, specifically, pressure. Because all of your super strength is concentrated in your tiny little hands, you're basically like a dagger plunging into a watermelon. Remember when Superman caught that airliner in Superman Returns? He'd have just gotten embedded in the nosecone. He'd be puncturing the plane, not catching it.
Same thing when you wind up launching yourself into space to stop that asteroid. At best you'll bury yourself in the surface of the rock, at worst you'll crack the thing into pieces, turning one killer asteroid into three. If there were any life on Earth left, we're pretty sure on your tombstone it would read something like "Here lies ______, who passed away from being metaphorically slapped in the face by Isaac Newton's penis."
"Also, in this country, we don't call people who overpower motor vehicles 'little girls.' We call them mountain gorillas."
—Gin (about Kagura), Gintama
"My voice gives me super strength."
—Tristan, Yu-Gi-Oh! The Abridged Series
"I got talent! Not everyone can punch their way through a mountain! Especially with their head!"
—Captain Marbles, MAD, "Superduperman!"