Because it takes a shitload of money to invest in the S&P500 and hope to earn a modest goal of say $500 bux per day. The average return on the SP500 is 10%. So if you throw $100,000 into the SP500, you'll make $10,000 per year before taxes.
Yet if you take your $100,000 and try to trade it, your earning (and loss) potential is unlimited. One could achieve $10,000 pre-tax/pre-commish in a year trading the ES and making just $500 per day on 20 trading days. 20 days versus 1 year to make same amount of money.
That is the allure. Potential to make a large amount of money on your own terms, not on the average return on an annual basis which you have no control of. And if you try it with $10,000, that other $90,000 could be used for other things. All $100,000 is not tied up in the SP500.
I think that is why.