Hello, because sunglasses or ski goggles are most important and common accessories for skiers. They do a lot helpful things to skiers. First it can protect the eyes from UV. What's more, it can protect your eyes from hurting by the flying snow, sun, wind and glare off snow. You are highly required to be prepared with a pair of sunglasses and a pair of goggles. Of course you will look more professional too.
Wearing sunglasses for fun may be one of the reasons. But I think the main reason may be that people want to avoid snow blindness, so they wear sunglasses.
However, I consider that's a bad idea. Because, it's quite dangerous.
We'd better wear ski goggles when we skiing or snowboarding. That's professional after all.