I think they aren't allowed to use the ones with holes and have to use relatively stiff ones, it would be dangerous to drop a Sharp or pointy thing and get it directly into your skin.
But they deal with many knives (and other sharp objects) and super hot liquids, and I imagine if you are working in a kitchen that serves a lot of people, you are also going to be dealing with some heavy stuff (eg. kegs and big pots)
Knives are sharp, but they aren't going to pierce a shoe, or a croc (without the holes), for falling down a meter. Also, they have to constantly move around they would get tired af with heavy shoes with metal plates on it.