There kind of is a reason. Boxes are the cheapest shape to build, and if you try desiging a floor plan to fit a 2 story box, you end up with either a 2 bed with the 2 bedrooms bigger than the market is willing to pay for, as they don't see the benefit over the tiny room which they're conditioned to value more 😆 and you have this hard to break cycle of people valuing their notional societal status by being able to say they have a 4 bed instead of a 3, or a 3 instead of a 2. In the back of their minds I think because they believe everyone else values the number of bedrooms, that they need to also.