> I enjoyed the rest of the article relating to the optimal dimensions of the cylinder, but I still don't really understand why more products don't use cuboids (with or without filleted edges).

Plenty do, when they are small enough that they can have sufficient strength and still have a pull-top opening (lots of sardine/anchovy cans are this way.) When they are bigger, they don't, because can openers.

Though I'm starting to see more "canned" vegetables/etc. that use lined cardboard cuboids rather than metal cans, so there's that.