Using just aluminum is inferior to using an aluminum alloy, which might be much harder. The trick is to find an alloy with something not very expensive.
They still are, including M109 howitzers and many other combat vehicles including some of the newer lightweight, air transportable tanks.
Btw, many navy ships are now being built of aluminum.
There is no such thing as “just” aluminum (unless you’re extracting ore) it is all alloy of differing concoctions. Even your beer can is an alloy specifically chosen for that lone purpose. I work in aluminum die casting, we use three different alloys for different parts according to the application and customer demands. There is no pure aluminum.