becouse that is the most basic answer that everyone thinks of...there for its funny lol wow you are the first person i know to even dare say they dont get it
It's just supposed to be stupid, cuz like ur expecting an interesting punch line, but you just receive the obvious answer. That's how it's SUPPOSED to b funny, but it's so cliched, that i lost it's uniqueness. that's my opinion, anyways.
Normally, you'd expect it to be some random or elaborate answer but it is something as simple as 'getting to the other side.' This is supposed to be funny, but really, it is just corny.