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.
.. until you realize that:
- they have extremely tight tolerances - they function together, a single defect one ruins the whole assembly - you have to make millions of them for cheap
Some things are deceptively hard. For a related fun fact, check out China's ballpoint pen adventures.
True. Though they've moved up in the world since then. They now have their own semiconductor fabs. Chinese industry of today is way more advanced than the China of the 90's making low margin nick-nacks.
See for example this video, how to choose your ball bearing
Now challenged by a few look-alike Chinese companies.
It's often easy to forget among all the noise but reddit has a lot of the best of internet too!