I think people just don't realize on gut level how fast driving is... someone mentioned Edinburgh as a place where it would never occur to them to drive. As it happens it was 5:30pm there when I plotted a random trip out of the city center to some residential area, it was 25min in "red" traffic vs 43min by transit, not counting overheads of waiting of course - so realistically x2 faster and that's from a city center. That was also my experience wherever I lived or visited, except rush hour Moscow where traffic is completely unreasonable and transit is good (although at rush hour very crowded, so reading is also questionable)... you'd look at "terrible traffic" and wince, then you look at transit directions and drive/call a cab cause it's way faster anyway :)
Which I'd wager includes a sizeable fraction of the population. I also remember seeing a study in Germany some years ago where they found that exclusive car commuters had the highest average BMI and bike commuters the lowest. Granted, it was not a massive (ahem) difference, but definitively a few kilos…
I certainly don't want people feeling like they're being forced into making inferior choices - just that we make an effort to design our towns and cities so that the choice to walk/bike/use transit is a realistic one that compares favourably with driving.