Airports and golf courses, on the other hand.. many uses for surface area besides living. I don’t know (truly) if the issue is really toxicity (in a well designed, reclamated landfill) or the simple fact that city services like sewer, water, gas, electric were never under-grounded there by design. You’d think settlement would be a cause for concern too, but I’d also think they have that solved if airports are made atop them.

Clearly you've never heard of Foster City!

https://en.wikipedia.org/wiki/Foster_City,_California

That's "landfill" in the sense of raising the ground level, not in the sense of "dumping trash", which is what we're talking about.