[Blue Ghost]

I think the reason is pretty simple; it's a fictional game. When cars were first made you either built them as a tinkerer, or they were custom made to your specifications. Later on Ford made them via the factory system, which cranked out more product for the masses.

Trying to justify your question is like trying to justify a fictional world where people in cars shoot at one another like fighter aircraft. It really isn't done outside of war or the odd criminal act.

So, if you have a society that embraces casual lethal violence, then you're probably going to want a design that's cheap and suits your needs. Not everyone in the CW-verse is a champion dullest with Gold Cross and the ability to earn gobs of cash. Most people are workaday folks who are eeking out an existence in a world that's suffered some major catastrophes. You don't have the resources to design and build your own personal car-tank, so you buy one off the lat.