The article poses the question but doesn't try to answer it. There are some plausible sounding answers here in the comments. Mostly that it is cheaper to do it that way. But wouldn't that be true elsewhere too?
I suspect a lot of it is down to consumer preference. Americans seem to favour big houses as a kind of absolute priority. If you’ve decided upfront that the house must be 3000sqft, then it will either be really expensive, or cheaply built. And Americans are more okay than most with the style of very low density remote suburban development that makes really big houses feasible.
