My homebrew world has even fewer races - elves being functionally extinct, and dwarves being decidedly weird. On the other hand, I lean towards a One-Of-Everything philosophy. Somewhere out there, there's an otyugh, or a shadow, or a ... whatever. Also, hardly anything ever conforms to what you might find in the Monster Manual. Another thing I've done is to keep civilization evenly matched by nature. There are only three major (human) cities, thousands of miles apart, with near-endless wilderness in between. I have plenty of space to explain how orcs, gnolls, centaurs and so on find room to live.

The only exception, btw, is dragons. I loathe dragons with the blazing intensity otherwise only found among the suicidally zealous.