shouldn't they be able to build reasonably priced large condos for big families that are not prohibitively expensive?

It probably comes down to the value of the land (which for condos is usually in downtown or dense areas and expensive - although I'm not saying they would need to be fancy and/or built in a downtown area) but compare a single building with ten 3000 sq. ft condos to ten 3000 sq. ft houses. But seems like condos typically cost more per sq. foot.

I assume it is a lot more efficient to build ten condos in one building than ten different houses - labor, material, utility infrastructure, etc. Outside of Utah where there aren't a lot of big families, I don't really see this being a thing, but seems like it could work in Utah in some places.