1. That's not necessarily a bad thing. It's hardly like everyone needs to live in an expansive single family home, and drive a car along a congested urban freeway to get to work. Even suburbs can be made denser, just by doing things like narrowing streets and reducing minimum setback requirements
2. Sure, there's enough space, but there's already stuff there, like lakes and deserts, which you can't build on unless you have an incredibly wealthy monarchy. There are also things like forests, which are valuable ecosystems, and religious sites
Conclusion: Unless you have the political will to turn America into 1 giant suburb, you might as well make them more liveable and dense