If we flip this and imagine that everyone knew with certainty what the underlying values (earnings, dividends, etc.) should be then the return on stocks would always just be the return on the underlying values (it is a bit more complex than this, in reality the cost of capital differs between individuals so you are always going to get volatility...but the point is that because people are trade their portfolios faster than fundamentals change, then there is always going to be a timing strategy that works...that is how HFTs make money most days over decades).