You are missing the point. The S&P 500 and most large mutual funds are not anywhere near as diversified as typical investors believe them to be and have become less so over the past few years. Investors buy them for the perception of great diversification as well as the ease of purchase/sale.

Diversity in number of holdings works well if the holdings are not heavily overweight/underweight and are not highly correlated. So RSP (equal weight S&P) would meet the definition of well diversified in respect to weighting. Unfortunately, equities have been trending towards an increasing degree of correlation which can't be adjusted for when an etf is constructed mechanically based on market cap.

I always urge friends, family etc. to look at the components of the various funds and etfs they hold as they will be shocked to find that, in aggregate, a significant portion of their position is in 10 or 15 stocks. For some that may be acceptable, especially if they hold other non highly correlated asset classes. For others, they need to take a little more time or consult a professional to research how to better balance that risk. As just one example, VXF attempts to capture the return of the non-S&P 500 equities.