You'd think someone would jump on this opportunity to produce a lower cost adjustable with a lower weight.
Hits the high points, wear on moving parts and tensile strength being chief among them. You could certainly build an adjustable from aluminum, and it's a very workable metal - it's just last on the list of just about every other metal in terms of physical properties or suitability for purpose with respect to an adjustable razor.Brass is self-lubricating with low resistance between parts, even after decades.
The strength of stainless steel avoids the possibility of breaking part details, and the surface does not need to be protected by electroplating.
The aluminum alloy must be corroded with an acid solution to corrode tiny holes on the surface, so that the anodic oxidation can be attached, in order to strengthen the surface and avoid oxidation; therefore, the long-term friction of aluminum alloy adjustable parts is worrying, and the anodic oxidation protective layer will be locally lost.
In addition, the metal rigidity of aluminum alloy is 2/3 less than that of steel, and the metal fatigue resistance is also very low, so the repeated force of the moving parts may accumulate deformation or even break.
If a brand were to make an aluminum adjustable razor, it would have to add thickness in many places and avoid a wear-prone structure; so the appearance would be different from any existing design.