Because the SP500 is a better indicator of the market than USD. Also if you look at the global dollar index, it is right at par historically.
The companies in the 500 are mostly global companies, if the USD shrunk so much they would either be losing money, or it doesn't matter because the US market is so strong it dwarfs the others.
> The companies in the 500 are mostly global companies
Isn't that saying exactly what the parent comment mentioned? Since those companies are global, the growth of the S&P 500 which is USD denominated will track the devaluation of the USD as the underlying companies haven't lost value, the dollar has, and the S&P 500 would track that as growth in percentage to balance it.
I don't understand why they would be losing money since as you said they're global, and more untethered to the USD than the S&P 500.