I think it's largely because we're still at the point in American politics where the folks against densification are still strongly tied to a certain pastoral vision of America: large lot sizes, single family homes, easy parking, and heavily manicured neighborhoods. It's more than just density, it's a legally codified way of life. Folks understand that the moment restrictions are eased that this pastoral view of urban development will necessarily change. It takes a lot of restrictions to have a built environment like America's and the only way to maintain that built environment is to resist any change to the restrictions.
But I agree. A vision of sustainable development would be a dense urban core with decreasing density away from the core, instead of endless SFH large-lot sprawl. Families or other individuals that want/need more space can live on the outskirts and take a train in to the urban core for work. Younger people or those who don't need the space can stay in the core, and folks in between can live anywhere in the spectrum of density.