I don’t have any background in finance, but what is stopping these companies from creating their own index and promoting it heavily? They have the market cap and the capital backing to do it I presume.
Because that doesn't force tons of passive investment dollars to participate. Being a part of the S&P500 doesn't require heavy promotion, they'd just get a slice of the investment from the millions of people who auto-invest money each month in retirement programs that buy S&P500 funds (among other things). There are already other indexes and funds where you can invest in high-tech/growth companies, but the people who are interested in that level of risk are already investing in them. These companies want to slip into the S&P because they'd get investment from people who are not specifically investing in high-risk/high-growth companies.