Well, smaller waterfalls make this general shape, decroded, and it's partially due to water's cohesion property. If you have a good faucet, turn on your faucet and watch. I also observed that general shape when I was around much bigger waterfalls that are probably ~4x my height when I sat down around waterfalls. The shape probably disappears if we're dealing with massive, massive sizes, but at this scale, I thought yeah, you should see the waterfall narrow down at the bottom.
(oh, just thought I'd mention that I also omitted the effect that the water "pieces" start to separate so you get literal waves of water continuously pounding on the bottom of the fall.)
I can see this is an obvious problem if we want huge waterfalls, but, who knows if we can accept a maximum waterfall size just for a smoother look?