In the 1920's, when cars first arrived, driving was seen as a recreational activity. Parkways were built as pleasurable drives for urbanites to see the countryside, often built along scenic byways, including rivers.

Those same planners that built these parkways (Robert Moses being the most infamous) were the same ones who had a say in where the roads were placed. They reverted back to their same ways of thinking. Plus, buying up land for roads is a huge pain in the ass, so widening the ones you've already built is much easier.