Stone Edge just happens to be the only powerful, common Rock STAB. ZenButt nabs critical KO benchmarks for a Medicham and Gallade, which also factors in. Crits and a chance of Flinch kinda offset I guess?Two that come to my mind are Stone Edge and Psycho Cut. Psycho cut has twice the crit chance of zen headbutt in addition to having superior accuracy, but it's base power of 10 fewer makes a huge difference in use. Stone Edge, on the other hand, is another move where I suspect the high crit chance impacts how often players use it, as it is the most-used move I can think of which only has an effective power of 80 by your calculation.

The calculation I posted was a rule of thumb nothing exists in a vacuum. KOes, PP, accessibility all matter.