I'm not saying that nobody dubs themselves, but if you watch enough Chinese shows that have been (poorly, usually) dubbed, you start to hear the same voices with different actors. Personally, I find it incredibly annoying. And creepifying for child actors. They always sound horrific.

Take it for what it's worth, but according to my Chinese-speaking friend, her theory as to why they dub is accents. China's a big country with many dialects, so they're dubbed with a standard accent. I do love that Chinese-language shows always have subtitles. Too bad I can't read Chinese...

The number one reason is accents. The second reason is to eliminate background noises. Especially in the case of Wu Xia dramas where they use big machines that make a lot of noises to make the characters fly. Majority of the time professional voice dubbers will dub as the actors and actresses involved in the drama would have moved onto another project, but occasionally the actors/actresses may make time in their busy schedule to come back and dub over their own lines. I think it's great when they do that. I've watched a lot of c-dramas and I'm very used to the dubbing so it doesn't really faze me. I also don't mind dubbing that don't match the mouth (in the case of foreigners). If you watch enough, you will get used to it. They usually do it very well. The words/dub voice may not match the mouth movements but when the voice finishes a sentence, at least the mouth stops moving. The timing is usually done well. I've seen bad dubbing in other languages where the mouth of the actor has stopped moving but the dub voice is still going! LOL.