It's called that because that's what it is. No South American, African, Asian, Australian, North American or Antarctic golfers can be on the European team.
So, although it is a multinational team, it is not a world team. And I don't know the specific numbers, but I bet the population of all of Europe is close to or less than that of the US.
I don't think the US is at a distinct disadvantage based on how the Ryder Cup is organized. Maybe just outclassed recently by better players!
If it WAS a worldwide team, to include Europe, I think the US would get dominated all day every day. Give the Euro team the world golf class, such as Louis Oosthuizen, Charl Schwartzel, Adam Scott, and maybe a Thongchai Jaidee... we would get smashed.