Because in the end he technically always wins. But that logic can be applied to any and all fictional characters with a happy ending. Batman has technically beaten far superior enemies before, but this was done with a deadly combo of extended planning periods (sometimes even months) and a hefty degree of plot armor. Take both of those away and Batman loses far more often. On top of that, Batman loses all the time, but only "wins" through the intervention of fellow superheroes. He's even considered to be the "most saved" superhero on the Justice League, making him the damsel in distress of the group--he just kicks a little more ass. So really he's only as good as people say he is in the minds of fanboys.