I think the reason why "you can't beat the market" is the simple fact that you are part of the market. If you are very, very good, such that any trade you make will always win, then the market just don't want to play any more. The feedback response from the market is extremely precise; do you make money or not? If you don't, you will change your strategy until you start making money or you just stop playing. The moment you start benefiting from some exploit, the market will immediately response to their loss by changing their strategy.

This is of course assuming that we are at a level playing field. I don't believe for one second that insider trading is not prevalent.