It's a coincidence. The only way to rationally manage diversification is to understand asset classes. Asset Classes Don't be fooled by the apparent complexity of your financial options. Other places to store your money are always based on one of these three kinds of investments. Cash, of course, is not an investment.
Most investors abandon some control and buy mutual funds, paying for professional management.
Even the segmentation of mutual fund offerings means investors face the dilemma of choosing properly diversified funds. Funds with broad diverse holdings typiccally charge the steepest fees, with asset allocation funds being most expensive, passing their research and churn costs along to customers as maintenance or sales fees. Different investment vehicles may be appropriate for different investors at different times, so for the individual, the only answer is to do one's homework. Read the fine print. Check that the assets in an investment vehicle are suitable for you.