Because they're funny on many levels.
You skip the punchline.
A corn snake
Nothing. (this joke was made by daughter when she was 5)
You're a little rougher than I'm used to, but I'll grit and bear it. I'm sorry, that joke was a little rough ;D
He who makes a mistake in an elevator is wrong on many levels.
Raise a family.
It was wrong on so many levels.
That would be wong on so many levels!