Because it's for your kid's bed. At 3 AM the previous night, they peed the bed, so you got the other one out and put it on, throwing this one in the laundry room. Then, today you washed it but the one on the bed already is still in good shape.

Or, you have sheets of a few different colors, each paired to a comforter with a different weight that is changed seasonally, or biweekly, depending on the preferences of you and your bedmate.