Yeah, there’s friction, just like there is friction from walking on marble steps, such as those in the Tower of Pisa. But since they are mostly moving with the object on top of them, wear is reduced. In the end, the little wheels will be working in concert to move whatever object is on them in a given direction, but they are round and so will have a bit of drift in short order, which is managed by putting the wheels at angles so only a portion of one side is touching the object. They are likely made of a moderate friction substance with high durability and are probably replaceable, just like the tires on your car.