That costs money. These ideas are all true. See my comments to Bill's questions below about the accuracies required. With the development of high precision non stretchable belts, there are some designs that use belts. The Paramount from Bisque is one example.
Belts show cogging just like gears.
There are some friction drives that use neither belts nor gears. Friction drives are not without problems. There are actually many different drive schemes described in the literature. With PEC and autoguiding you can get sub arc second tracking. Also, GEM manufacturers like to brag about how big their gears are.