Because superman is what you get when you let a retard decide their own 'OC's powers with no limits. Superman is overpowered to the point that he becomes able to use his powers under any sun, eventually gaining immunity to his only weakness, Kryptonite. Also, iirc, there was a point where he had to sneeze, so he popped into an alternate timeline to let it go, the force of which destroyed a galaxy.
Batman, though he was born into money, didn't get the advantage superman gets. The money helped hugely, but he still had to put a lot of work into being able to fight crime without getting his ass sliced up every time he came across a villain.
TL;DR: Superman is practically unbeatable by birth while batman has to keep sharp to stay ahead.