Dividends, like increased stock price, are just another equivalent way for shareholders to gain value (with some tax differences). Many investors reinvest the dividends and so never "see" them.
Of course there are many market indexes that we can follow, but why is there more attention on the S&P 500 than the S&P 500 with dividends?
Another point: Inflation affects the real value of stocks. If I am simply comparing two investments in a given year, I can ignore inflation, but when I track an index over decades, X% annual growth in the 1970s compared to Y% in the 2000s is meaningless if one ignores inflation.
So why is there more attention on the S&P 500 than on the S&P 500 with inflation?