To me, I see Europe as the center of global motorsport, hence why so many drivers either come from here, or come to here from other continents. America however, I think has it's own bubble of motorsport, with Indycar, NASCAR, etc. So understandably, American drivers are more likely to stay to this bubble.
Agreed, though I don't think Indycar plays into it that much. I would argue that Indycar loses more talent to stock car racing at the grass-roots level than anything else. In the end, Indy car doesn't have any talent that F1 teams would want.
They're a special case because there aren't huge scores of Canadians, Japanese or generally South-East Asians making it to F1 either. Only recently have we had several Australians popping up but they struggle too.
I'd say Brazilians tend to get quite a bit of backing because of the legacy of drivers like Emerson Fittipaldi and Ayrton Senna. There has been a strong connection between Brazil and European junior formulae for a very long time.
But Canadians, Japanese, Australians and South-East Asians do get their chances in lower formula. For the most part, they don't impress enough to make it to F1.
It is easier for Europeans to get seats in lower formula, but that's only natural. The whole process starts in karting followed by start-up formula, and these are mostly run locally, so it's obviously easier to get local kids and kids from neighbouring states into the karts/cars. Look at this year's ADAC Formel Masters series, 20 of the 21 drivers are Germans or are from Germany's neighbours. By F3, FR2.0 and GP3, there are more non-Europeans coming over, but by this point, costs are much higher and so only the most well backed non-Europeans are making the trip over. Often these are among the most talented young drivers from their countries, though that's not always the case.
There's also the issue of local grass-roots motorsport in the countries in question. Canada doesn't have a strong motorsport heritage (Villeneuve family aside) and the Indycar ladder is an option for young Canadians. Japan and Australia have their own motorsport ladders, though they aren't as well developed as those in Europe (I would guess karting participation is quite low in these three countries, especially Japan). I imagine South-East Asia and India don't have very well developed karting programs. These countries (perhaps Japan aside) aren't good comparisons with the US either, the US being a large developed country with a strong motorsports industry.
NASCAR a threat to F1? I don't buy into that argument. In fact to say that NASCAR is threat to F1 would be like saying that NASCAR is the primary reason why MotoGP and AMA is failing to make a splash in america. Not even MotoGP legends Schwantz and Rainey blames NASCAR for the lack of american participation in international motorsports.
NASCAR isn't a threat to F1, it's a threat to the creation of American F1 drivers. It's undeniable that there's driving talent in the US, but most of it goes into the NASCAR ladder system. It's the smart thing to do. There's money in stock cars in the US and I imagine there's more support and it's cheaper to run than single seaters. If NASCAR wasn't the behemoth it is, there would be more talented kids going to into single seaters and eventually coming to Europe for F3/FR2.0, but that's not the case. Instead, the US is sending over duds like Michael Lewis and Jake Rosenzweig.