Sorry average anatomical humanoid being, not everyone is born with an inherent force-field built into their chin. We all don't hail from the same planet as Nick Diaz, Dan Henderson and Rampage Jackson. If genetic code operated on the same mechanics as a role-playing game, their DNA dumped all of its skill points into the construction of their chin. It's impossible to max out that level anymore. They've hit the apex, now they can spend the rest of their life using their chin as earth's primary defense against asteroids.
Earlier this month at On Top 5 in Kelvin Hall, Glasgow, Scotland, Jessin Ayari was under the impression that the material his chin was constructed of was made entirely out of adamantium. Leave it to undefeated welterweight, Alan Johnston, to ruin that illusion. Jeez, what a party pooper. Props to Ben Cartlidge for the find.