So let's say all or even most wrestlers have that mentality. Which ones rise to the top, to win national championships, become all Americans, etc? The most athletic ones. Those are also the ones you see transitioning into MMA. Most fighters in the UFC have worked very, very hard for a very, very long time. Athleticism is a huge separating factor between 20-10th ranked guys and top 10 guys, and wrestlers who have made it through the grind of high school and college wrestling in the US have it in spades. You're starting with a very, very broad base of participation and only the most successful and athletic guys are going to stick with the sport all the way through college. The pyramid is much more narrow for BJJ, kickboxing, and (in the US) Judo, so the guys at the top have not had to overcome the same kind of opposition as elite wrestlers.
When everyone works hard and everyone is mentally tough, only the best athletes are going to rise to the top. Those are the wrestlers we see transitioning to the UFC, and they're head and shoulders above the rest in terms of athleticism. And it's a huge factor in their dominance because it lets them pick up other skills so quickly and do things that other fighters simply can't do from an athletic standpoint. Yes Dan Cormier works hard and is a great fighter, but think about how quickly he's picked up striking and submissions. Look at what he did powerbombing Dan Henderson. His speed and explosive strength are off the charts, and a guy like Glover Texiera is never going to match that no matter how hard he works. Cormier just has a higher top end, and on average anyone who's made it through the crucible of American wrestling (or Russian or Iranian wrestling, for that matter) is going to have that athleticism because otherwise they wouldn't have made it through. The gulag doesn't make you tough, it just kills off anyone who isn't tough.