It’s only because in MMA, because the combatants start training at a very old age (20-30), it is almost like a Russian Roulette. They are learning MMA on the job with very few amateur fights, so anything can happen. It is watching fighters learn in real time.
In boxing. There are amateur tournaments for kids, there is a much larger talent pool that gets eliminated when they turn pros. There are vastly more tested professionals in boxing. In MMA even at the highest level the participants start training for MMA between the ages of 19-30, this is normal. In boxing you can not get away with such little experience. Many MMA fighters only have 3 or 4 amateur fights and then turn pro, in Boxing it is normal to have 50-250 amateur fights. The pathway in boxing is much more difficult as it is closer to a well organized professional sport where participants must start at a young age in order to gain success.