I think it's important to recognize that the development of a city has a LOT to do with the time frame in which it grew.
Big cities like NYC and Chicago, smaller but older cities like Philly and Boston, grew to become what they were before cars were ubiquitous. Density wasn't a design "choice", it was the necessity of people having access to commerce. Walking and public transit were how you got from point A to B.
City planning and city development changed once we got to the post-war and baby boom eras, when cars were not so much a luxury as something any middle-class family could afford (even if they only had one per family, unlike most families today). It also corresponds to a quite large racial change as we got into the civil rights movement and post-Jim Crow era, where there was a lot of "white flight" out of the cities to the suburbs. It was also a big time of people moving from the farms and rural communities closer to city centers, and many of those people preferred the less dense suburbs. And finally with the interstate system, national infrastructure was starting to be developed around the automobile.
You started to see cities that were less large and established (i.e. most of our 2nd-tier cities today) grow around the automobile. This would also be true of cities that aren't "2nd-tier" cities like Los Angeles, more due to the time frame in which they saw rapid growth. There is a "downtown LA", but it's not a "downtown" in any reasonable sense compared to cities in the eastern US corridor.
In the first case, density existed before the automobile. In the second case, the automobile led to cities being less dense.
Now everyone wants to reverse that in order to force more density--on people who don't want it. You know who likes dense walkable cities? Young childless couples. Give them a few rugrats running around and suddenly they want a SFH out in the suburbs, with big parks, and bike lanes, and room to breathe.
