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

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

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

The human hand naturally curves into a circular shape when closed.

Squeezing anything in a hand will naturally form a ball shape. This will occur with anything, but paper is more readily available and crumples easily.

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.

You have been deceived. People always crumple paper into a flat disc shape. All evidence showing ball-shaped paper must have been fabricated.

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