Markets are dominated by a few large investors, creating problems of concentration. Similar portfolios and strategies exacerbate risk and the problems of illiquidity if a large number of participants or very large holders wish to exit positions at the same times.
Investors are frequently market following trading the momentum, buying when prices go up and selling when they fall. They are users rather than providers of liquidity. Their buying creates the illusion of active trading when markets are rising but suck liquidity out when prices fall.