Beside very cheap products, the only aluminum parts I have found in my kitchen tools are either injected or molded. Why this?
Because the fusion point of aluminum is much lower than steel, plain steel molds can be easily used. The opposite is not true and for steel casting, it requires disposable sand molds. Thus, the process of steel casting is more complex and way more expensive.
I realized that it is not easy to find stainless steel injection molded parts apart from very big stuff such as car rims or machine frames. Almost all steel parts are either machined, laminated, forged or heat pressed, but never injection molded.
So engineers will use stainless steel or plastic for any part that can be simply laminated, forged, heat pressed or injected. However, for more complex parts the only remaining choices are: injection molded or CNC machined. The latter is too expensive for high volume production for both steel and aluminum.
Steel injection modeling is neither a solution because of the process complexity.
KitchenAid is probably focused on cost reduction and will choose the most affordable. Using aluminum is the best compromise in terms of cost because aluminum casting is not much expensive and offers good mechanical performances.
Making the same part in stainless steel would make the part five to ten times more expensive. It is worth it, just because you can safely put it in the dishwasher? Certainly not...