But then you wind up with a portfolio that isn't balanced and isn't tracking like an index fund. An index fund doesn't simply buy a flat amount of stock and hold it, they buy stock in proportion to the relative weight of the exchange. Which is always moving
Market cap weighting is special. If company A has 500 shares, company B 500 also, than a fund that has 5 shares of A and 5 of B is market cap weighted.