*lol* I don't need to. It's the same reason why the TIE fighters have no shields. They Empire prefers swarm tactics! No, seriously, if you ask me, that's the Empire's solution to unemployment. You can't make the bills, you want to eat, then you put on a suit of white clam shell armour and heaven help you if you're a non-human because it's slavery instead.
That's not to say that the Empire doesn't have their elite troops. You're going to see a few of them in Rebels now that the rebellion is shaping up. They've already shown two inquisitors.
Speaking of which, the question I want an answer to... is why is Captain Rex on a remote planet during Star Wars Rebels and why wasn't he affected by Order 66? He's in the trailer for season 2 which doesn't premier until most of the way through June.