Something I've never seen on this board is a discussion about using aluminum to make a cannon or mortar...So here goes. ...
Good question.
One issue is that not of strength, but of the characteristic of the metal itself. 'Toughness' or the resistance to being worked and not becoming brittle and breaking is very good in cast iron and extremely good in forgings (to include aluminum). If I'm not mistaken it has something to do with the grain structure and alignment of the larger molecules.
Go one further than aluminum - look at carbon, synthetics and so forth.
You see fiberglass reinforced tubes for lower pressure applications as in the LAW (light anti-tank weapon) rocket launcher.
I've seen .22 barrels with a minimal diamter tube and ends of steel filled in with carbon fiber for strength - and light weight.
We KNOW pvc pipe isn't good - very little strength where it counts.