Because while snow is soft and fluffy when it first falls, after a few temperature changes and having been groomed and skied on for hours/days, it transforms from snow crystals to ice crystals. Unless new snow falls on top, that's what you're gonna be skiing on, and you'll slide all over the place without sharp edges.

Here is one of the best skiers in the world, on a race course set on a surface intentionally prepared to be solid ice: https://www.youtube.com/watch?v=lIHdHUg1FC4