This was just a way for home builders to save money by building fewer walls.
They convinced homebuyers and influencers that is trendy, that living in a house that feels like a Walmart supercenter was the thing to do.
I really don’t think many homebuyers asked for a toilet next to their living room.
Unless you’re talking about a really big span – and usually we’re not, since even open floor plan houses tend to be long in only one direction – it’s not that big a difference. A little bit of extra wood for beams and columns is cheap compared to the overall cost of the building.
Retrofitting an open floor plan on a building that wasn’t initially designed that way is expensive, of course, but that’s a different thing.