OK, I will add one thing.

For all the talk of US airlines having hubs here and there and everywhere, let's not forget one thing:
AA is now, but US, HP, PS, PI, EA, TW and more were.
UA is now, but CO, NY, PE and more were.
DL is now, but NW, WA, NE and more were.

Expansion into more and more hubs in USA also largely took form of buying out other carriers - the difference is that by and large in Europe, it makes more sense to maintain the (national) identity and AOC of the absorbed airline.