It's European invasion

inna-
good query. The companies that you cited realized that their counterparts are junk-sourcing to china. There are a few American originals still left.
There are a few Japanese originals left as well. Excellent points of view as above.

Economic argument is a good but not the only one. And it may not be as straightforward. If we buy more of what some make now and keep buying it, what economic reason would they have to innovate? They would have more financial resources, but would they channel enough of them to R&D ? And what about those who do have motivation and knowledge but cannot establish themselves because of lack of funds?
It also appears that Europeans manage to make good sounding equipment at all price levels.

MHO....the innovative companies are the ones that thrive on feedback, that care about their products, and know that happy customers tend to let others know how happy they are.  Too true, not all survive, even given the above, despite a superior product.  We've seen examples of this too often...
We make our choices, and vote with our dollars.  And hope the cream still rises instead of merely being an oil slick. 

Hundreds of companies in North America make great audio gear. Decware stands out in my mind as a quintessential example.