Aluminum cases were developed to save money for those volume buyers who did not reload, specifically police, federal agencies and the military. Since aluminum is not as elastic as brass, reloading aluminum cases can create problems with cases cracking, so the makers of aluminum cases made them to be non-reloadable. (Some folks have tried getting around that, but it is a lot of trouble and not a good idea anyway.)
Saving weight was never a consideration, since aluminum cases were not intended for other than range use.
Jim