That risk can be either diversified away or parceled out and sold. This proved to be untenable, mainly because the very nature of risk is still ill-understood: the samples used in various mathematical models were biased as they relied on data pertaining only to the recent bull market, the longest in history.
Thus, in the process of securitization, "risk" was dissected, bundled and sold to third parties who were equally at a loss as to how best to evaluate it. Bewildered, participants and markets lost their much-vaunted ability to "discover" the correct prices of assets. Investors and banks got spooked by this apparent and unprecedented failure and stopped investing and lending. Illiquidity and panic ensued.
The short answer is: because it is far more convenient to get involved in the market through a fund than directly. Another reason: index and tracking funds are excellent ways to invest in a bull market.