I don't think that's motivated by money. The US companies simply solved more interesting problems. Working for a start up in the Bay area trying to invent a new industry, or scale systems to global is generally more interesting than working on a CRM system for mid-size lumberyards in Sweden. The CRM system pays well enough to have a comfortable lifestyle and provide for your family, but it's a little boring if you're 25 with a shiny new CS degree.