my theory is because the "new breed" fighters (the ones that start to train mma instead of being specialized in a single discipline before cross training) are rushed to the ring and they just didn't have enough time to develop a proper skillset, fighting against other non specialized fighter make them feel safe with their limited skills and at some point they are too busy to prepare for a fight that they don't have enough time to work on the basic.
with the few exception of the rule, obviously, but imho in the last 15 years it looks to me that the mma gyms tend to send young, unexperienced fighters to the ring after like 6 months of training (imho less than 3 years is criminal).