Well, they're in the Russell 1000 and others, not to mention a bunch of ETFs that are basically indexes of their own. If you want a properly diversified global index fund portfolio, you wouldn't want to limit yourself to 500 of the largest American companies.
Mine contains 4 funds covering ~2500 global companies.
You wouldn't know them; the funds themselves are Norwegian. IIRC, the reference indexes amount to 50% MSCI World, 16% MSCI Emerging Markets, 16% MSCI Small Cap and 16% VINX Benchmark (Nordics).
Philosophy is diversification, broad coverage of both developed and developing markets, with both high-tech niche industries (heavily overrepresented in the Nordics) and global growth companies (overrepresented in the Small Cap index). Been considering specifically adding China, but dislike the political risk and human rights of top leadership. You might argue that the USA is still overrepresented in this allocation.
I was probably underestimating the number of companies covered by this; the Small Cap index alone represents more than 4500 companies. Although I doubt that my funds actually own all of them.
Mine contains 4 funds covering ~2500 global companies.