The basic idea is that if you do better than the market - because you have more of the good stocks and less of the bad ones - there may be someone somewhere with the opposite positioning.

In this simple model active investors in aggregate cannot do better than the index. Adding transaction costs, fees, etc. they'll do worse - as a group.

A reference (that lays out the statistical arguments) is "Common Sense on Mutual Funds" by John Bogle.