A passive investor makes money one way: the market goes up.
A systematic long/short approach is built to do something different, to extract returns from movement itself, in either direction.
Let's walk through the three cases:
1. Market up: the long sleeves capture the trend, much like passive.
2. Market down: here they diverge completely. The short sleeves profit from falling prices. Where the passive investor just absorbs the loss, a long/short portfolio has a sleeve that can generate returns from this regime.
3. Market sideways: A range-bound market goes nowhere for the index, but it still swings within the range, and mean reversion strategies harvest those swings. Again passive exposure will end up with nothing, while a mean reversion sleeve will profit.
That's the structural advantage.
Passive needs one specific outcome to win. A multi-sleeve, long/short portfolio has a way to profit from up, down, and sideways.
It will not beat a roaring bull market every year, that's not the goal, and any honest manager will tell you so.
The goal is to not be dependent on the one outcome passive requires.