The problem of course is the setting can only cover so much. There are ten races in the PHB and 20+ in the MotM. How many pages of the hypothetical Greyhawk book do you want explaining each of those to you? What happens when a new race like dhampir or hadozee are released?
So far, WotC has opted for basically three types of models when dealing with the races in settings:
1. Closed. A list of common races with the option to allow the DM to add more via travel. (Dragonlance, Theros, Ravnica)
2. Kitchen sink. Common races are explained and everything else is just assumed (Faerun, Wildemont, Eberron)
3. Multiverse. Touches every setting, anything goes (Planescape, Spelljammer, Ravenloft).
I see nothing wrong with any of those approaches