Well yah it isn't the ONLY reason they rich. Like the US their wealth is mostly attributable to being early pioneers in the Industrial Revolution and obtaining a massive technological lead overs that still persist today. That doesn't subtract from the fact that much of their wealth came from colonial exploitation. While Europe today doesn't exert the same level of direct control over Africa as they used to, companies can still go into Africa and do pretty much whatever they want by bribing local officials.
Another thing that I forget mention regarding Europe is that they basically get a free lunch with military spending. The US doesn't. The only way we could possibly have Social Democracy in the US without going completely bankrupt would be to drastically downsize our military. We can't do that because that would mean that spooky socialism and other foreign threats would begin to proliferate and threaten the Western world order.