Just like a bearing, gears (ideally) have a rolling contact. The smoother and harder the faces the better, until you get to the point where fractures and spalling occur. The best gears are cut, sent out to be hardened via heat treat, then ground (with "superfinishing") to exact size. As with bearings, you have to get the size just right for long life.
See for example this video, how to choose your ball bearing
Now challenged by a few look-alike Chinese companies.