I'm sure everyone has their own reasons, many being symptomatic of the image-obsessed country we live in.
For a start - there's just the fear of doing anything different; it's not just cycling, I despair at how many people will NEVER use a bus, they just see it as below them. I have a son who's just passed his test, goes everywhere on his bike but as soon as he can afford a car that'll be another one lost. To him it's part of a) growing up and b) being 'successful'. I was the same at his age (20).
The fear of public exertion is also strong; even now I usually avoid the only significant hill on my route because of the fear I won't get up it; and while it's a long drag, it's only a bump by national standards. Yet it's perfectly OK to get off and walk!
Of course when I started commuting getting up that hill was an impossibility; that first few weeks while your own fitness builds up; discovering what clothing works; getting the bike set up right (most non-cyclists will start with the saddle way too low); getting used to bike maintenance etc. You can just jump into a car and drive.
Now - as a cyclist and motorcyclist - on the odd occasion that I drive anywhere I find it tedious, and really hate the whole business of finding somewhere to park the thing. I don't complain that there's no parking; I find an alternative. I've seen the light, but most people would rather moan than do anything about it.