There aren't any separatist groups because everyone knows that China will grow larger either way, destroying whatever small progress to independence they may have made.

In all seriousness, there are so many Han people that they can just move some people around to wherever they want and it becomes sturdily Chinese. Also, such separate ethnic identities seem to mostly be a rural, not-very-modern thing so it simply seems to make more sense to stay a part of China, so long as that China is still stable to some degree.
If you want various groups to break off and form their own real states, you have to get the tendency for strong Han centralism to die off or really fail hard. A prolonged Japanese occupation or more aggressive Western quasi-colonization efforts could achieve this effect, but China, as a very large nation, seems to be hard to "keep down" insofar that it struggles to determine its own path, which has always favored authoritarianism.