I hear this a lot, but it's really not true. Most "pizza places" are small businesses (<50 FTE), so they aren't required to offer benefits anyway.

They want part time employees because it offers scheduling flexibility. If all you have is full time workers and someone calls out sick, you're short a person. If you have mostly part time workers, you can call everyone not scheduled that day to see if they can come in. Office jobs don't really have this because it's typically not critical that the employee is there.