People act dumb both in groups and individually. You don't see very many intelligent people joining big groups and becoming moronic - they're smart enough to recognize groups cater to the lowest common denominator and avoid them.

The exception being young people who have the potential to be intelligent but lack life experience.

As much as you'd like to believe that people are smart somehow - the average IQ is 100. 100 is really, really dumb.

If that's true, why is it so hard to beat the market in the long run.

It's better to say that experts are better at some things and crowdsourcing is better at others. Though that's neither snarky nor pithy.