The premise behind a lot of urbanist thought seems to be that there’s a fixed quantity of things you need to access, and the reason you drive is that those things aren’t close enough together. Therefore, if provided with a dense enough neighborhood, you won’t need to drive.
This is the idea behind the “slow transit is ok if there’s enough density” argument that folks have been using to defend slow streetcars and LRT. It doesn’t matter if the transit is slow if there’s a lot of destinations that are close enough together.
I think this misunderstands the utility of driving, and indeed, the utility of transportation in general.
The purpose of investment in transportation is to make it possible to travel more distance in less time. It’s not to provide access to some fixed quantity of things you need, it’s to increase the quantity of things you can access. This is true for both low-density and high-density areas. The difference is that in high-density areas, you can access more things for a given investment of time.
For example, you could live in Koreatown and work downtown. These are among the densest neighborhoods in LA, and you’ll have an incredible amount of amenities within walking distance. But if you want to hit the beach, you’ll have to go to Santa Monica or Venice. If you want awesome Chinese food, you’re going to have to go to the San Gabriel Valley. Or maybe you have friends that live in Hollywood that you want to visit. Or maybe you want to take a class that’s only offered at UCLA or CSU Northridge. Or maybe you just want to go walk around a different neighborhood for a change!
Slow transit doesn’t help you with any of that. And here’s the thing: even if K-town and Downtown get so dense as to offer all the amenities just mentioned, other parts of the city will inevitably evolve to offer different things. That’s just how cities work!
The appeal of driving isn’t that it makes it possible to access the things you need in a low-density area. It’s that speed makes it possible to access more things at any density. If transit is going to compete, it needs to compete with speed.
Let's Go LA 
Even if K-town and Downtown get so dense as to offer all the amenities just mentioned, other parts of the city will inevitably evolve to offer different things. That’s just how cities work!
I feel like a lot of people, including but not limited to New Urbanists, don’t seem to get this. They tend to think of a city as a bunch of villages that happen to be located right on each other’s borders, but don’t seem to realize that the total is larger than the sum of its parts. And the benefit of the city is that there can be all sorts of things that are interesting only to a small minority of people and thus require a large population to support. Of course, you can try to convince people who share a similar interest to all live in a village dedicated to that interest, but that interest tends to be something along racial/ethnic lines, we call that a ghetto.
I agree; that’s a crucial difference that’s often left out of the discussion. A traditional rural small city or town center can be walkable and self-contained. But even if you could get all the people who have a similar niche interest to live in the same place in a large city, they’ll all still work in different neighborhoods, have other interests, have friends/relationships in other places, etc.
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The title of this post ought to be “do people misunderstand the utility of transportation?”- nothing is exclusive to automobile transport in this post. It could just as well be about bicycling, HSR, or hoverboards. Increased mobility has the purpose of getting you between one location to another more quickly. Increased accessibility has the purpose of getting you to the maximum number of destinations in a given amount of time.
As other posters mentioned, cities are greater than the sum of their parts. While many or even most days you may remain within Glendale (population 200,000), that’s not like living in Gainesville (similar population). You have access to the city.