So what, neither would a basketball player. They are two separate sports, with different rules so you train and use your body an use weight in much different ways. The best athletes go where the money is so why would an excellent boxer take the time to learn MMA?
If you are trying to make the point that MMA fighters are better overall fighters, like in a street fight, I would dispute that always being the case. In a fight with no rules someone willing to bite and gouge could overcome much of the wrestling or Judo based aspects of MMA and the winners on the average would be the better striker. This is especially true if there are multiple attackers.
An ex-con once told me the difference in a street fight and it wasn’t the particular skills of the combatants. He said the meanest person wins.