Of course, nobody can predict the stock market and nobody can always find good investments. In most professions, this daily diligence would be rewarded. In the world of Wall Street, this behavior is often punishing. Most of a successful investment's value is created at the tail-end of its lifetime. You need to hold a stock long enough for this to occur.
Many managers are too focused on quarterly returns.
Fund managers also naively jump in and out of holdings in hopes to maximize returns. Before even accounting for taxes, this active buying and selling almost always works to their detriment. The desire to perform better than peers also leads fund managers to invest in high risk, unfamiliar territory, or strange asset classes. Hundreds to even thousands of funds are doing this right now. They invest beyond their normal competency, dipping into overseas investments, commodities, hedging, and more, all in hopes of earning a much higher return.