Because policies they cause huge financial harm to normal people become unpopular, and don’t spread as a result. And also because it is bad for society if we modify the rules in ways that makes it harder to plan ahead.

It doesn't really matter why; what matters is that they do see things that way.

And I agree with you, it's a bad convention, and it causes tons of problems, and it's the result of recent decades of bad mortgage/housing/zoning policy. But we can't just wipe it away and pretend it was never there, and let the chips fall where they may. That's just heartless, and, well -- you talk about making society poorer -- that will make society much, much poorer, nearly overnight.