The biggest reason is that such fans are designed for low pressure and high volume operation, when you blow air around the room you really don’t need very much pressure.

So the fan blades already are already no longer operating at their optimum pressure when they have 5 filters to suck air through (each filter only has to let 1/5 of the air through, so the differential pressure over the filters is much lower), but it’s much closer to the ideal operating pressure than two filters behind each other.

If the pressure gets too high the fan stops being able to push any air through the filters and the efficiency reaches zero.