It's not common
But, some artists lip-sync because they want to sing & dance at the same time, and are not capable to do that. They want to make it more entertaining by dancing so they choose to lip sync. Not all artists are capable of singing and dancing, only a few like Lady Gaga ect. & Also I think it's because their probably insecure of their live voices, messing up, or missing a note. Most of the time I don't mind when they lipsync