D&D has never been about creating complex, believable societies, or emulating historical or even fantastical reality. World-building in D&D (and this is especially emphasised in 3E) is really about _setting a stage_ where the actions of the characters can be played out. Everything in the DMG is subordinated to this task. Hence the extensive treasure tables, but no lists of day-to-day wages for various occupations; the random encounter tables but no discusion of ecologies or geography; and so on. You don't need to know how much a bartender makes per month to slay dragons, nor do you need to know how many cows/precious stones/virgin sacrifices that dragon eats.
I understand your point, hong, and I agree that the game doesn't need to dwell on earnings, tax rates, etc. On the other hand, setting the stage is much easier when you have some idea how a typical ancient or medieval society works. How does policing work in a medieval city? How does the military work? What happens to a group of adventurers who show up at the city gates?
If you want to set up some political intrigue, it's good to know who wields what power, and what they might be fighting over. How does money work? Are there bankers? How do they protect their money? What's a merchant caravan like?
I don't think I could've answered any of those questions as a kid playing D&D, but that info would've helped me set the stage for our heroes.