I think the reason why it hasn’t happened in the U.S. is because land is not at a premium here. Look at where they reclaimed land (or emptied water out) in Asia and those countries aren’t exactly huge land masses and are relatively crowded in terms of people. The ones I can think of off hand are HKG, ICN, KIX, MFM and now the new airport in Dubai, Jebel Ali. With the exception of Dubai, the rest of those places are not large, square mileage wise and reclaiming land was the best option. I will address the 3 cities you mention.

Los Angeles: They have room for expansion down here. Sure, they have to move a few things around, but LAX is still built on a very large area of land.

Chicago: Two airports, one of which has significant traffic, but again, ORD is built on a huge mass of land. They have so many runways and surely have room to expand if needed.

New York: Three major airports eliminates the need to seriously expand any of them, seeing as they really spread the wealth in terms on traffic (none of them handling more than 40 million pax annually). Now, if they wanted to ditch the three airports and build one massive one, then taking the existing JFK site and reclaiming a significant amount of land from the water adjacent to it building one massive airport would be feasible. Thus creating the largest airport, in my opinion, in the process, undoubtedly handing 100 million pax annually. But this isn’t in the works by any stretch of the imagination.