This isn't really the issue. Most of that lower nation-level population density is due to the fact that the US possesses some truly vast wilderness and wasteland within its territory. I'm no geographer, but I'm pretty sure there's no equivalent of the Nevada desert contained within France somewhere, let alone Alaska.

The real issue with the US is suburban sprawl. The population isn't too spread out on a national level; it's too spread out on a local level. If the US had European-style urban design, the existence of some vast empty wilderness won't affect things. We're talking about how the people in the inhabited parts of the land live. The wasteland is irrelevant.