As long as we're fully omniscient,This is not a requirement. In fact, that's the beauty of what the market does for you. All you need to know is your own utility function (and you are likely not even aware of that, it comes automatically), and the price (i.e., salary and other compensation). Friedrich Hayek won a Nobel Prize for demonstrating how pricing in a market distributes information, communicating the priorities and substitutability for everyone.
there are no costs in making changes
Of course there are costs in making changes. How could there not be? Surely you're not suggesting some magical world where everything is always magically altered to the optimum. Any time anyone is confronted with a choice, part of the decision making process is to consider the costs of changing. If that costs exceeds the benefits from the change, then you won't do it. So I don't see what you're trying to say in this phrase.
we're all perfectly rational
The rationality of actors is, admittedly, a matter of some controversy in modern economics. We know, for example, that people are systematically bad at handling values of extremely large or small scale, like the actual risks from terrorism -- we've badly overreacted to something that overall poses vanishingly small risk.
However, in this particular discussion -- a person's benefits from their employment, fully considering their job satisfaction -- then I don't think that anyone is in a position to debate. There is absolutely no way that anyone can second-guess another person's utility function (that is, their values and preferences, and how much a given thing improves their quality of life). We simply cannot know what is happening in another person's mind. Indeed, we frequently don't truthfully know our own motivations. Yet it's still clear that these preferences do exist, and that we take action on them. When talking about something this micro, we have no choice but to rely on a person's observed behavior. (For much better discussion of this, see Mises' Human Action).
good enough!
Well, yeah.
As an alternative to sarcasm, perhaps you could articulate an alternative explanation to my own. The only other that I can see is a Platonic top-down control, with a group of blessed decision-makers deciding what the relative values of everything ought to be. But there, you've still got the problems of omniscience, as well as forcing all participants to adhere to the same set of values, rather than allowing each of us to make our own choices.