This is a very good point that I see often. I haven't watched a ton of MMA but I have watched enough to see many knockouts that happened for this very reason: low hands.
In my limited experience with Wing Chun, I feel that there is a connection between the elbows and solar plexus region, and power is transfer to the arms from this lower cage. in this way strikes, blocks, and trapping come straight from the center, getting inside where attacks can occur that are not allowed in sports. In Boxing, the shoulders lead the punch, and twisting delivers power from the hips, and this more naturally coincides with a higher defensive cage.
Also in boxing the range of targets are limited, and lower body attacks are not expected. the lower arm positions in many martial arts are because they were developed to fight against other martial artists, who kick. In a fight to the death, knockout is well and good, but the strikes are often aimed at the throat not the jaw, and a higher cage could even expose the brachial artery on the underside of the upper arm as a target. Likewise, a lower arm position will be a mistake against a skilled boxer.
I often say that every MMA competitor needs to put in their time in a boxing gym, and every martial artist needs to at least study boxing long enough to take it seriously. A skilled boxer is no joke, boxing is powerful, and very sensible. adapting traditional movements to modern martial styles requires a flexible discipline, and that is what MMA is really doing now as traditional martial arts adapt to modern relevance.