> The S&P 500 brochure describes itself as "the best single gauge of large-cap U.S. equities". That language implies they act as a benchmark,

Okay, but isn't that gauge measured over a specific timeframe? Since investors in this index have timeframes in years/decades not days, why would you expect the index to have a ranularity of days?

> That language implies they act as a benchmark, which I find questionable, given that based on their current eligibility requirements, it would exclude all three of SpaceX, Anthropic, and OpenAI.

Sure, but it excludes lots of companies. This specific index is risk-averse and caters to risk-averse investors; regardless of whether the company is SpaceX, Anthropic or OpenAI, rick-averse investors are going to shy away from any share that hasn't been traded long enough for price-discovery to kick in.