Despite producing a lots of well known Grand Master why China haven't produce a single Mixed Martial Art ?

Ancient martial arts is not designed to be mixed. Each style is passed down only to disciples of that clan/school. Disciples are faithful to the style they use and don't go around to different schools learning all styles (unless you get kicked out from that clan and have nowhere else to go).

If you want mixed martial arts, you'll have better luck watching Jackie Chan. His martial arts is more mixed as he has to make it "interesting" for western audiences.

Yip Man was a wing chun master. Wing chun is not a mixed art. It's 100% chinese.

Bruce lee invented jet kun do which is a may be argued to be a hybrid martial art, but I am not sure. wat's it mixed with?

Champions? well, look at the last olympics. I believe Chinese got gold in judo and boxing.

Jeet kune do is a mixed martial art invented by Bruce Lee who is from China.