Most likely because the gravity on Krypton is about 100 times that of Earth's. This would mean that Superman's bone density and muscle mass is far greater than that of Earthlings. His flying ability is because of this and is similar to when astronauts go to the moon and can step about 10M each step.

Cos hes alien. Batman,spiderman etc are all human. Superman is not. he was born that way on krypton then came to earth

Because DC comics decided to make him do those things naturally.

Because he's a cartoon character