Because the fundamental purpose of an index is to track the stock market. The S&P 500 benchmark was created in 1957 to benchmark the US stock market, decades before the first investment funds that copy it (by Vanguard, in 1976).

The primary purpose of an index is to track the market. If an index excludes a significant part of the market it claims to track, then the index no longer accurately reflects the market, and fails at achieving its purpose.

The S&P 500 tracks the 500 largest large-cap US stocks. All three of the major upcoming IPOs (SpaceX, OpenAI, and Anthropic) are large-cap US stocks. Together these companies comprise ~5% of the total US stock market.

In previous decades, this was not an issue, since companies IPOed much earlier when valued at <0.1% of the market. It was fine to exclude these companies from the index for some time, since they were an insignificant part of the market.

Today we have companies raising enormous amounts of private equity, and going public as significant members of the market. All of (SpaceX, Anthropic, OpenAI) are within the top 20 largest companies in the US.

This is why many are arguing for fast-track inclusion, so the index can add these companies quickly, and retain its ability to accurately track the market.