Most investment returns are random

This is the main reason you should use index funds. The majority of stock picking is pure random luck. There are good years and bad years but if you think the fund manager has control over this you are kidding yourself. Investing is one endeavor in which a complete amateur can destroy a professional over a period of time. This could not happen if there was not a tremendous amount of randomness.

From Josh Brown

Past performance, in particular, can seem to be a plausible basis upon which to form an opinion. This is because we are programmed to recognize patterns in nature and to extrapolate what we believe we have observed. However, studies have shown that there is a high degree of randomness in relative investment returns and that to be statistically significant, a performance record should be intact for nearly 15 years. Few investors meet this criterion. Fewer still meet this requirement and have not experienced other changes which have a direct impact on future performance, such as staff turnover, or growth in assets under management, which can affect portfolio construction. Consequently, I strongly believe that – considered in isolation – past performance is a poor basis for assessing an investment.