Because a single PE teacher can manage five rotating classes of twenty kids throughout the day. If kids didn't rotate periods, then all 100 kids would take PE at the same time. That would require extra PE teachers, because one teacher cannot manage 100 kids. Multiply this effect by every different area (music teachers, art teachers, etc.) and you have to hire a bunch of extra teachers which is a waste of money. Plus, those teachers would only be busy for part of the day, so their wasting a lot of their time.