Capping the Interstates

It seems like something out of the future but why hasn't the federal government or cities for that matter thought about capping all of the freeways with new buildings and streets to gain revenue? It seems to me that in areas which have urban freeways and high land prices, we should be able to not waste that land. Why not build steel tables on top of the freeway and just build up? There wouldn't be too much objection to it and perhaps there would also be room for transit on top.

We already see ideas for either capping or submerging freeways and this might be a way to just leave them as they are while using the air above them as buildable space. And to make sure that the freeway isn't an eyesore from street level, there should be shops or housing wrapping the sides of it.