It doesn't.

You would need a LOT of plastic.

There is a reason almost all drink containers are square, except for pressurized ones which are universally round.

It's pretty much impossible for it to be actually square. It would need to bulge either in or out.

Take a look at the bottom of a pressurized bottle, and notice it's not square either.

I don’t understand the downvotes. I wasn’t trying to imply anything in particular about the amount more plastic, or where the trade-off actually sits. Indeed, my naive guess would be that soda vendors aren’t idiots, and probably currently do whatever is most efficient.