At 34, I've just bought my first car. I live in a scandinavian country. Up to last year, my daily transit distance never exceeded about 6 km one way. IOW, I've lived all my life in a fairly compact urban environment, and public transport has been both more convenient and cheaper for my needs. The same has been true for the vast majority of my peers - most people I know do not own a car.
Now, I live about 12 km away from my office. There's a direct bus line, which is about as fast as a private car, so I've stuck to public transport. It's become a bit of a hassle though, what with having a kid, and some trips taking an hour due to having to change buses etc, but doable. Just don't go to certain places, it takes too long and the kid will remind you just how long it's taking all the way there.
And finallly, just now, I'm moving out of the city, and have concluded it's time yo cave in and buy a car. Wow. So much easier. Most people who live where I still do now, within very reasonable public transport distance of the city center, seem to have cars. I see why, now. But I managed to live over 30 years in areas where public transpory was, hands down, the best option, and I think it simply comes down to how far I needed to go each day. Over 6 km, it becomes a bit of a hassle. Over 12, at least one car in the family is probably in order. And once you have the car, you're more likely to use it.
I'm guessing a lot of people in Denver have cars and need to travel at least 6 km one way on a regular basis.