There are places that zone for conservation. There are also organizations that buy land and place deed restrictions against developing it, then resell it.

There are areas around me that do these as well as allow for high density housing. These areas are suburbs, small towns, and small cities in the Philly region. We still have sprawl, and prices are relatively high. Even though it's possible to build high density housing in many areas, most people don't. They can get more for large sfh since there's a lot of demand for that. There are some apartments and condos being built, but it seems most of them are pretty expensive or upscale. So even though the zoning is permissive, sfh are the primary construction due to preferences. The conservation can also inflate prices by restricting supply and introducing regulations to deal with.

Just an aside on the regulations... some of them can be extremely stupid, like a prohibition on impermeable surfaces being interpreted to include stuff like gravel. If gravel is an impermeable surface, then maybe we can take the regulator to the desert and pour their water on some gravel and come back in a few days...