It comes from an economic theory. The problem is that people who say that kind of thing ignore one of the basic assumptions of that theory: that all actors have

It comes from an economic theory. The problem is that people who say that kind of thing ignore one of the basic assumptions of that theory: that all actors have all the necessary information to make the best choice (and also that they act "rationally"). The problem is that it is very rarely possible to obtain that information, and even if it is possible it might take a lot of time and effort.