When you cook polenta, you are instructed to cook it until it 'comes away from the sides of the pan'. Why is this when starch cooks at around 100C? Is it to evaporate some of the water, or to make sure the individual 'grains' of polenta are soft and cooked?
If you try some when it's only just come to a boil, it's pretty obvious that it's both of the things you suggest. The cornmeal won't be cooked (it'll be gritty, still half dry, not soft) and there'll be too much liquid still.
You could of course use less liquid, but that wouldn't change the fact that polenta, like most other food, doesn't cook instantly. It needs some time to soften and absorb water all the way through.