You can buy some pretty cheap on EBay form China and they look pretty good, too

They usually consider 58 cm "Extra-Large" and the geometry is more race than relaxed, but no reason to doubt they are quality low-budget aluminum frames.

And no, manufacturers don't see any profit in selling bare Al frames because anyone who is willing to spend the money to build a bike frame up is probably looking for something a little special. There just isn't much market for it.

I wanted to build a touring/gravel bike which could take a rack .... but pretty much no one else wants that. People who want to tour go for steel, people who want to grind gravel seem to not care about racks.

So ... I bought a decent off-the-shelf bike.

Why exactly do you want an Al frame? Not to be critical, just to understand.