I believe...

The US economy is based on spending as much possible as a consumer. Rather than focusing on infrastructure to improve livelihood, the aim is have endless construction and maintenance for as many people as possible. Everybody needs a car, or else you cannot safely travel(in the normal-ass suburbs where I grew up it was an hour walk to the grocery store, few pedestrian crossings for highways, and there plenty of sidewalks that simply ended due to gated communities or neighboring towns or villages that simply didn't to build some twords there edges). Each car will need to be taxed, registered, sold, bought, insured, fixed, fueled, towed, fined, accessorised, and finally scrapped and sold again in peices. If there was good public transportation, then less money would be changing hands, and the economy would look less impressive..

I don't think that was original reason, but I'm certain it's on the minds of modern economists now