I read it in a book. It is quite standard.

And I'm pretty sure that the people who first invented it were driven by modeling motivations, rather than experiment. Mathematical techniques already exist to solve maximization problems. The first field which really looked at the issues in a systematic way was microeconomics - and this kind of model is the kind of thing that would occur to an economist. It all fits together into a pretty picture; most of the unrealistic aspects don't matter all that much in practice; bottom line is that it is the kind of model that gets you tenure if you are an Anglo-American econ professor.

Really and truly, the motivation was almost certainly not "Is this the way it really works?". Rather it was, "What is a simple picture that captures the main features of the truth, where "main" means the aspects that I can, in principle, quantify?"