We don't, actually. They used to have to vote themselves raises. Then they tried to get automatic raises voted into law. The public went nuts. So they waited a bit and Dick Gephardt slipped it through before it could get anyone's attention. My congress critter at the time.
I didn't know Gephardt (that miserable failure) did that. Last I knew they voted in their own raises. That makes me even madder that it is now "automatic."