Why so many frequency changes?

On a recent flight between Phoenix and San Fran I was listening to ATC com between the pilots and ground and they must have changed frequencies 20 times. What's with that? Is that something new?....or was it because of the route we took? I think they changed frequencies several times just in the Los Angeles center area alone. I'm sure the pilots were just on-board to spin knobs on the radio and didn't have time for anything else. We did land though...although I don't know how they found time to take their hands off of the frequency knob.