The short version is that it is much easier to form complicated shapes in aluminum than stainless. Easier to form and more processes to do so.
On the materials side, When you connect dissimilar metals, their is a potential that forms between them (long topic). This drives redox reactions at the surfaces of the two metals, and the stable species depend on the pH of the water they are in. In this case the aluminum will preferentially dissolve into solution. If you must connect dissimilar metals, you want the surface area of the more Noble (i.e. SS) to be small, and the metal that corrodes large. This is because you actually care about current density (current/area) on the part that is corroding. Here you have a lot of surface area of stainless, which, in broad strokes, is probably not a good decision.
On the materials side, When you connect dissimilar metals, their is a potential that forms between them (long topic). This drives redox reactions at the surfaces of the two metals, and the stable species depend on the pH of the water they are in. In this case the aluminum will preferentially dissolve into solution. If you must connect dissimilar metals, you want the surface area of the more Noble (i.e. SS) to be small, and the metal that corrodes large. This is because you actually care about current density (current/area) on the part that is corroding. Here you have a lot of surface area of stainless, which, in broad strokes, is probably not a good decision.