The S&P 500 is used as the benchmark of the market by practically everyone. Journalists, policymakers, investment managers, politicians, regular investors, everyone I know. If the benchmark that everyone uses as a market proxy is systematically excluding a substantial part of the market, then the gap betweeen "the index" and "the market" has real consequences.
You can't have it both ways: Either the S&P 500 is a market proxy, in which excluding parts of the market is a problem; or it's a curated slice, in which everyone needs to stop it as the default benchmarket for the market.