The United States is ruled by spreadsheets and algorithms that the banks using them can not comprehend. From their perspective, 'it seems to work' and they base policy around these spreadsheets.

The simple way it works is that within every county, there is a branch of a bank that actively collects data from the county. They keep track of how many people live in the area, how much square footage is appropriated to each home using openly available data from the county registrar, what the rate of growth is for the county from hospital records, and they all have a policy of busting down any large scale developments that would create lower returns from their investments in existing housing stock. They want prices to be as high as possible for as long as possible. I've personly had a long held suspecision that NIMBY protests are frequently organized by shadow banking groups.

So you input the data, where the county lines end, how many residences there are, how many people live there, etc. and you can basically 'adjust' the economy around your needs since people can't get their county to approve any more housing permits than the bank would let them. Its total rule by spreadsheet.

So why have the banks allowed loans for mcmansions outside of the suburbs? They're on the fringe between rural areas and suburbia, and rarely in cities. These are shoddy homes that are built far too large for American families, who are trending downward in size, and who tend to not have multigenerational families under one roof. What could be the expected return on investment here? This messes up their spreadsheets!

There is a general plan to sell them to foreign buyers to get them to move their entire families into the U.S. This is the conspiracy. They can be but like crap because the house will be bought site unseen by a foreign family. And they're large because the intention is to have a move-in by a large multigenerational extended family.