The core issue is that S&P requires companies to be profitable for 12-months to get included in the index. Yet all of SpaceX, OpenAI, and Anthropic are highly unprofitable, because they are prioritizing investing all free-cash-flow into growth instead of returning money to shareholders. These companies likely will not be profitable for years, and without a rule change it's unlikely they will be included in the index anytime soon.

Given these large-cap companies currently represent ~5% of the U.S. stock market capitalization, it's difficult to justify why these companies are excluded from a large-cap index.