Because throwing a paper ball into the bin( or the near floor) is far easier than a flat piece of paper.

Because it’s too hard to crumple a sheet of paper into a cube.

IANAL, but I think it’s because balls are fun to play with.

It serves two purposes; easier to throw, and harder to read. You do it to things you don’t need anyone to read.

If you get frustrated by something you’re writing, crumpling it into a ball gets out frustration by destroying the work and then it’s easier to toss into a garbage can.