maybe when the patent expires in a few years this will become common then.
Funny that I didn't see one from surefire. They should be cleaning up.
They've made a couple using mode selector rings, but not infinite dimming. The UDR Dominator does have an off position on its dial, but that's not intended to be the main switch. You don't see those often, as the market for $1200 flashlights, especially relatively outdated ones is small.
There is one: the Jetbeam RRT-01, at least certain iterations of it. Most designs use a separate power switch.
This isn't more common because there's a patent covering using a Hall effect sensor in a ring around a flashlight body for this purpose, and the patent holder, Surefire has sued several manufacturers and vendors over it. A well-funded challenge would probably succeed on obviousness grounds, but patent challenges are expensive and I don't think the flashlight industry is profitable enough to justify it. The patent expires in three years, so this will probably become more common then.