This Doghouse Diaries comic pretty much explains why we only have one set of bed sheets. If we had more than one, we'd have to figure out some way to fold the spare. Considering how little time we have on this Earth, we don't want to waste it learning to bend time and space until we enter a dimension where the laws of physics allow the folding of a fitted sheet to be possible. Before you say we're gross for only having one set of sheets, know that we wash them sometimes (monthly?), and you weren't going to sleep with us anyway so pipe down.