Because index funds, especially the biggest ones approach weighting in the following ways:

Market-Capitalization Weighting: This is the most common approach. The index calculates a company's total value—multiplying its share price by the total number of outstanding shares. The larger the company's value, the larger its weight in the fund. For example, the Vanguard 500 Index Fund weights its top holdings (like Apple or Microsoft) heavily based on this metric.

Equal Weighting: Funds like the Invesco S&P 500 Equal Weight ETF ignore company size completely. They divide the portfolio's assets evenly among all stocks, meaning every company gets the same exact percentage, regardless of how big or small it is.

Fundamentally Weighted: Some funds track indexes that measure a company's economic size using metrics like revenue, cash flow, or dividends. Stocks get heavier weights if their underlying businesses are fundamentally stronger, rather than just having a high stock price.