Dragons and giants generally keep themselves in check. The reds battle the golds, the frost giants battle the fire giants, etc. They're too busy concerning themselves with their fellow big-nasty-powerful-guy rivals to spend much time conquering smaller folk.
Aboleths, mind flayers, and others just have too many enemies keeping them in check, as well as their own kind's machinations and double-crosses. And most importantly, ADVENTURERS keep them in check. Usually. When they don't, that's when other adventurers do the job.