Currency rates mainly, also GDP doesnt mean wealth is equally distributed.....
The top 20% in america earn 8 times what the average do.
The top 20% in europe on average only 4 times.
This inflates GDP on US side but doesnt give an overall wealth picture fairly.
Also look deeper....compare average incomes in different states of the US as you are doing in Europe...
Look at NC (very average state) more representative of the US then salaries in New York or California (NC comes in about 40k, while New York and Cali much higher. But Purchasing power is exactly the same with the lower salary in NC!
There are more in depth factors to consider then just GDP per capita because it doesnt show you who is wealthy and who isn't.
Also alot of US wealth is simply financed...and creates an illusion.