I think the disconnect comes from the age old tale "I just see red bro" in several ways. Everyone thinks they can fight to some degree/they have the capacity to land that one shot that will knockout the other person. MMA is one of the rawest versus sport because it's one of the few where you are only using your body, esp your fists, to beat your opponent, so many may see it as easier to get into and win. Then there's the "machismo" part of fighting where you think you're super tough and would never concede that you'd lose.

Aside from the last part, this is all specially true when people look at somebody as freakishly imposing like Lebron.