"Leaves everyone better off" (i.e., making a Pareto improvement) is a tall order if there are more than a few people involved. Just "making the pie bigger" is much more plausible, but in general people will disagree about what counts as making the pie bigger, since we don't have an agreed-upon way of doing interpersonal comparison of utility.

I'm not saying that discussion/debate can't serve the purpose of joint optimization, but it often doesn't, and we can mostly see why.