Agreed, there are even aluminum scuba tanks. The issue with a steel tank is the potential for rusty air on the new smaller tanks fitted to OIL FREE compressors. As you stated, on oil feed units you might get a coating of oil on the inside and of course drain daily. As I'm buying a small 1 HP, 2 gallon unit for mostly blowing "clean" air, the aluminum tank seems like a plus for my needs. I read an entire web site mostly devoted to why one should never buy an aluminum tanked compressor, and metal fatigue etc. Many of the points seemed logical, and since at first I could not find any, I assumed NO compressors were made with them. More searching proved that wrong. In the current catalog of California Air Tools, most of the smaller compressors have aluminum tanks.
Now I need to find a good quality air blow gun, looking at two models right now.