Literally a couple decades like 20 years? I just don't think that could be true, at least not where I live.

Looking at the listings in a broad radius around me, there is still a lot of housing built in the 1960s and 1970s being bought and sold quite regularly, and some stuff even older. A house built in 2004 ("a couple decades") would absolutely not be worth the cost to rebuild.

I'm curious where in the US you're from, or where you've seen that. Florida coast? In the cities I've lived, most of the houses were built right after WW2 (80 years ago), and in some older neighborhoods, most of the houses date to the victorian age. I can't even imagine someone doing a teardown on one built within the "last couple decades" (i.e. built in the 2000s).

The vast majority of houses aren't rebuilt with any regularity in the US. The extreme high-end McMansions are commonly rebuilt, but that's an outlier. Much of the affordable housing is the luxury apartments of 40-80 years ago.

I would suggest that the UK is also similar.

It's just that we don't do it because of housing shortages, or the ever popular terraced housing, and people live in some of the worst states.