The manager decides which stocks to buy and sell. Fund names can be misleading. The name of a particular fund does not necessarily reveal the types of stocks favored by that fund. Funds change investment strategies. Even when you know what the mutual fund invests in, the fund manager can change direction at any time without seeking your approval.
Managers come and go.
If your fund manager leaves and you're stuck with an inferior replacement, you'll face taxes if you sell shares to invest elsewhere. Fund holdings overlap. While you want diversification, you could wind up owning the same stocks in different mutual funds. Funds grow too large. When thousands of other investors buy the same fund, the manager is flooded with cash.