What significant building has there been in America in the past 30 or 40 years, compared to 100 years ago? And especially in the past 6 years?
Where is the next New York? The next Chicago?
Where is the imagination any more?
Ok, on a somewhat serious note, your questions are not very well stated, but I think you're asking why we don't see development in America at the same scale we saw in the early 1900s.
Quite simply, why would we? The industrial revolution lead people into urban areas. Once people we housed in urban areas, those urban areas continued to grow. The 'next Chicago' is probably San Francisco, which is still fairly small compared to the influence it is having on industry.
The last 6 years have seen a massive downturn with the GFC, don't expect to see many large developments happening during a real estate crash.