While in school I spent 7 months in a co-op for a company that manufactured steel food cans. Although at the time I never challenged the need for circular cans I can say that manufacturing would be much more challenged for cube cans.

Metal likes to be formed by rolling, and it's relatively easy to image how cans are cut to length, passed through a die and seam welded. Obviously that's a lot of capital for one can size so round dies are easily swapped for various size cans on the same line. Forming stamped and bent edges would be much more intensive for changeovers.

Lids would require directional placement: Round shapes fit in all directions, pretty clear here that any other shape would require it to be directionally correct.

Can liners are sprayed: The inside of your cans are coated to protect the food, corners are harder to maintain an even sprayed coating

There are plenty of others I'm sure and this is a rambling post but maybe it gives a little more insight into the world of cans.