It's one of those things you've probably never even bothered to think about, so think about it now: just why is a mouse cursor a slanted arrow and not, say, a straight arrow? Or a pointing fish? Or something?
The answer is a simple one. Go back in time to the birth of the mouse and we find we're dealing with displays with a very low pixel count. Douglas Englebart's very first designs for a mouse cursor had it pointing straight up, which on those blocky monitors made the pointer tough to spot.