The interstate system, started in the 1950's, pulled a lot of business out of small towns and closer to the interchanges. Also, the rise of suburbs from after WWII contributed to a lot more sprawl than in England, for instance. We just have more space to spread out!
Many small rural towns, especially county seats, still have a "town square" and the courthouse close by, but the coming of Wal-Marts and big box stores like that pull business outwards to the outskirts. It's nice to see downtowns revitalized, though. In the days before automobiles, of course, people would bring their horses and buggies and tie them up outside the buildings. But like someone else said, "Downtown" of a large-ish city tends to be mostly skyscrapers, old churches, and historical buildings. Little Rock, for instance, began with the Statehouse right on the river and mushroomed outwards.
What's interesting is the difference in city planning from East to West in this country, from those areas that used to be the colonies, out to the "West".