I guess we disagree which one is the primary driving force. I see NIMBYism as the primary driving force.
When it comes to real estate location is one of the primary drivers of cost. Here's
a 600sqft 1BR/1B condo in NYC that's listed at $125,000 and here's a
2,288 sqft 3BR/2B house In West Virginia that's listed at $129,000. Demand to live in major metro areas is what is largely driving up prices. Of course many developers will focus on luxury units but that's partly because of delays and costs imposed by excessive environmental review and community oversight meetings which make it such that only luxury developments are viable.
I personally know of at least two cases off the top of my head of homeowners who wanted to add an additional unit on their SFH. One for his elderly parents and the other for their adult children. The former was barred and just didn't do it and his parents died and the latter did it anyway and had to pay a fine when code enforcement came around. Sure many neighbors might not want that but I think its silly to argue that people leveraging the government to ban the building of certain home types is the market at work because if those folks were allowed to they would've built and kept the extra units.
And if that is the argument then it still falls apart as we're seeing zoning reform across the country to allow for more density in R1 suburbs such as laws to relax codes around ADUs and multiplexes. Some are even streamlining the building of such units with preapproved designs. This didn't come out of nowhere, its because of the housing crisis that these reforms are happening.
People in the market to buy a SFH are going to look for SFHs. Adding multifamily units to suburbs is a way to add units to the rental market for those who would like to pay less for the same amount of space or otherwise live in the suburbs but either can't afford to buy a SFH or otherwise don't want to. Its also a way for those who own SFHs to increase their property value and create an extra income stream without having to buy an entirely new property. People looking to rent such units are those who are lower income(which includes those from middle and upper middle class families who are starting their careers) and those looking to buy them are those with extended families that want to live together. In Canada part of the reason such houses were banned was because immigrants from Asia were renovating their SFHs into multifamily ones as their children became adults and the family expanded. This is common in Asia, in fact my dad paid to have such a house built for his parents and siblings back in India. But that would be illegal in most places in the US and Canada.