It you don't establish the rudiments of a base setting, then the game becomes a generic system like GURPS. D&D is a fantasy game. You can change anything you want (and there should be rules in the PH that encourage you to do that), but you have to have something to change it from.
I'm not saying that the game should be generic. I'm saying that game should not exclusively discourage race/classcombinations. It doesn't have equally support some combinations. The game should grant some advantages for choosing some race/class combinations.
Not every setting has tribal orcs who have no wizards, it shouldn't be baked in the system. No it doesn't have to support orcs switching their bonus from STR to Int. However it should give a wizard a noticeable benefit for having 14 STR.
Setting should subtract, not the game itself.