That's exactly my point!
While it's technically true that Indy always has had its share of foreign drivers, many people equated Indy with American racing, not international racing, especially in the 40s, 50s, 60s and 70s. Guys like you mentioned, plus of course the Foyts, Unsers and Andrettis, etc. Indy was as all-American as apple pie (at least, with the vast majority of drivers being American in that era, it felt that way). But that's no longer the case.
Again, I'm not suggesting that the dominance of foreign drivers in Indy today is necessarily a bad thing, but it does make the Indy 500 something a bit different than it had traditionally been, in the eyes of many people. Not all, of course... but many. Myself included.