Cost, I don't see why or how it would take up space, but I do have a deep bilge
too, so maybe they aren't well suited for a shallow bilge
Boat I looked at buying
had one, it was if memory serves essentially one valve with a manifold and the items requiring water
, drew off that manifold. But it was like a bucket welded to the hull
, so knock off that valve or bust the manifold etc, the bucket would fill but as the top of the bucket was above the water line, no water would get into the boat.