It's because intelligent people understand the truth: the market can stay irrational longer than you can stay solvent and in the long run we're all dead.
There are no guarantees (other than maybe death and taxes). There's only the likelihood that as long as the market is rational you will make money in the long run. As soon as you start setting odds on success, it becomes gambling.
Investing in the market at-large, though, is based on so many presuppositions of general global order that I struggle to understand what the entire market being “irrational” over an investing period of 30-40 years would look like short of a new world war. And I mean that sincerely and without hyperbole.
There are no guarantees (other than maybe death and taxes). There's only the likelihood that as long as the market is rational you will make money in the long run. As soon as you start setting odds on success, it becomes gambling.