Because they don't stop moving. All that flailing does is punch holes in the water that you sink into which creates a nasty feedback loop of thrashing causing you to sink followed by even more panicked thrashing desperately trying to reach the surface which causes you to sink even faster and so on. Treading water requires a specific stroke which floats you on top of the water, or you simply wait and float up to the surface, but to float up you need that air in your lungs for buoyancy since you are heavier than water otherwise and you're probably going to lose that air from panicked thrashing. No air in your lungs, the harder it is to float, and the more filtered the water the harder it is to float in it since there's less particulates to help suspend your weight, that's why the Dead Sea makes people so buoyant for instance.
TL;DR: Humans aren't meant to float and it's pretty much impossible if you panic