I'm not sure that airliners are optimised for anything really. They seem to be a mish mash of design ideas, that nobody has the bottle to break out of. If they did, then the current ideal would be made to look very old very quickly.
A rounded fuesleage cross section is not really any good for interior space or load lugging. Mind you the only square section a/c I know of is the mighty Shorts 3/30-60 so maybe not the best example!!