I was about to comment the same sentiment, but to answer your question... You also need to think of this in the context of convenience and arrogance from the point of view of the high-modernist, technocratic planner of the mid-20th century. By that point, there was so much pollution and degradation of the natural elements of our cities' waterways, that many US cities thought the easiest and most efficient way to get value out of them was to do with what they had and felt was the cool, new thing: the automobile. With deindustrialization slowing rotting away - literally and figuratively - at the core of our waterfront infrastructure, a lot of these docks and wharfs were just vacant real estate. It can be easy and efficient to build something that costs a lot, like highways and automobile infrastructure, on land that is "vacant" instead of occupied. (Not that they didn't do that though! See, the Highway to Nowhere.

EDIT:

Provided some helpful links for further reading if anyone is interested.