Why do we pay taxes for our sidewalks?

Lawrence Lessig, while talking about how internet access in America today is slow and expensive, said “[i]f you rely upon “markets” alone to provide infrastructure, you’ll get less of it, and at a higher price.” His point in the item is to suggest that, contrary to the arguments of free-market proponents, things like roads and powerlines and fiber-optic cables work better when the government acts to encourage them. Leaving aside the question of whether internet access is really analagous to interstate highways, I’m curious about Lessig’s assumption that government regulation and investment in infrastructure work better than a pure market served by civilians (public and private corporations, presumably). While I suppose it could be true, I don’t really see why. Is it because there’s limited physical space for infrastructure, or that it’s inefficient to have companies competing for utilities traffic, or that there’s a high risk of collusion in any market for such vital services? Or, more likely, is it something else entirely?

Hopefully some of you smart people can help me out with this.

Also, go Redskins.