When you're trying to put furniture in the room, trying to fit something with a rectangular base in a room with a rectangular plan is much easier than any other similar problem while minimizing the likelihood of having any wasted space.

Plus, it's also much easier to change the configuration of the room, or to transfer one piece of furniture from one room to another. So even if people weren't just lazy about how to fit perfectly pentagonal furniture in their houses, so long as they were trying and testing new shapes, furniture would probably evolutionarily converge to something squareish.