Almost all houses are built by the builders for the development, so they're all building the same damn house over and over and over again, and since they don't know who the customer will be, they build the default "acceptable house" as currently looking good in Home magazine.
Try to find a new house without an open concept. It don't exist.
A lot of it is practical. People tend to live differently than a generation ago. How many people today really want a formal dining room that's separated from the kitchen? Or a dedicated formal living room?
A number of years back I had to have some major construction for structural reasons and it ended up opening up much of my ground floor considerable--and it's really so much better.
I'm not saying it all has to be one big fully open floor plan but generally speaking at least kitchen/dining area and maybe other common spaces are more useful as an open plan.
Hmm… I have no need for having both separated formal dining rooms and living rooms, too, but I also don't want the kitchen's smells, fumes and heat entering uncontrolledly into my living room, either.
Try to find a new house without an open concept. It don't exist.