Most of wrestling and jujitsu and the clinch hold in Muy Thai boxing are isometric and dynamic strength or dynamic tension meaning that the muscles are contracting against each other for strength. I do exercises from a book I have titled “Dynamic Strength”, and I do karate katas or forms while stiffening my entire body throughout. Doing the katas with dynamic strength allows the muscles to be worked in all positions throughout their full range of motion. From how I feel doing these exercises and katas I believe they are very effective at building strength and explosiveness and not as much bulk as weight resistance exercises. So why aren’t isometric and dynamic tension (also called dynamic strength) exercises a bigger part of mma and wrestling training?
I'm familiar with what you're talking about because in old Kung Fu styles we used to do "Iron Shirt" exercises which seems to be exactly what you're talking about.
Without personal training qualifications or anything of that nature I'll attempt to tackle the subject just from an experience standpoint.-
I would assume that it has something to deal with lactic acid buildup and the development of specific muscle fibers. You want to teach your muscles to operate efficiently with the exchange of O2 and CO2, and also lactic acids. A lot of fighters do high rep exercises to train their muscles to keep operating at peak performance for long periods of time while increasing the exchange rates of all the aforementioned things. Dynamic strength exercises work for this to a degree, but I'm not sure if they work as well as the high rep exercises. Specifically because the muscles in a dynamic exercise cannot simulate an explosive movement which engages different muscle fibers.
For the muscle fibers that dynamic strength exercises would work it is probably better accomplished during live rolling sessions. Nothing will get your muscles burning quite like live resistance from an opponent while you're attempting to lock in a kimura from the bottom, squeeze a guillotine or a triangle choke.
All that being said these are just my assumptions. I've trained in both traditional styles and modern MMA for several years and have been a gym rat for a few years as well. I've got experience doing all these things, and from what I've gathered so far there is nothing that trains your body for MMA quite like live sparring/rolling.