There didnt use to be belts in karate when it first started now there is. Martial arts are changing all the time maybe one day mma black belts will be a common thing. Mma itself did a lot to change martial arts conceptions so you can't say if it isn't broke don't fix it because if people had stuck with that attitude martial arts would never evolve
Of course I can say it, boxing doesn't have belts nor does wrestling so why should MMA have them just because someone wants to make a bit of money on grading fees? Karate was 'invented' for want of a better word for entirely different reasons than MMA. Many people think karate hasn't evolved but has rather gone backwards and become a shadow of what it's supposed to be.. empty hand civilian self defence. For many point sparring and the emphasis on children's classes leading to 10 year old black belts has tarnished karate, gradings have little value in many places so why MMA should have gradings to satisfy those who think a black piece of material around their waists makes then a fighter I don't know.