Sometimes simple things bother me….

In a dense urban environment, in cold climates, why are sidewalks at street level?

Warm air rises. Walkways elevated above baffled entrances (to keep out the wind) should be warmer than exposed sidewalks, and always dry. Elevated walkways across city streets can only improve the flow of traffic. Given the huge investment in large buildings, why not also improve the public space associated with the building? Should enhance the value of first-floor tenant spaces as well.

What are the arguments against?