Not all precuts are high quality fabric. I have gotten some really bad ones - not cut of straight of grain and/or fabric is thin. Yardage comes in all qualities, high to low. Precuts are not washed because fabric is not washed AFTER cutting. It could go wonky and then not be usable. After it is sewn together there is less problem.
I prewash yardage for several reasons: 1) to get rid of any chemicals in the fabric, 2) to make sure the grain is straight. It can be hard to straighten fabric with sizing in it if it has been stretched on the bolt incorrectly. 3) To make sure it is colorfast, and 4) I prefer working with fabric that is clean, starched and ironed. It cuts easier and straighter and goes together easier.