It's good that they don't, because they would get beat decisively. I shudder when thinking of how brutal it would be to watch Jon Jones destroy some poor clueless Tau Chi practitioner who thought they had a chance.
The traditional martial arts that are actually effective have already been incorporated into MMA. Jujitsu, Muay Thai, Wrestling, even some Tae Kwon Doe, Karate, these have all made inroads. But nobody makes it far these days in MMA knowing only one form.
Even if the best tai chi guy could hold his own on the feet, the MMA fighter would just go for a takedown or pull guard and the tai chi guy would not have the tools to defend.
There is not a single person who practices Tai Chi who has anywhere close to the breadth of martial arts mastery that someone like Demetrious Johnson has.
I do and I've met serious people who can do some incredible things. These people are serious and don't go into the ring with some self promoting idiot.
To be fair most MMA champions don't even have the breadth of martial arts mastery that someone like Demetrious Johnson has.
I believe that's called religion.
I'd love to see that video.