I've played about 30 hours and haven't seen one yet. I'm level 23, do I have to be a higher level to get more of them?
Nope. Completely random. Best to buy the items using Vanguard points or the equivalent if you're desperate. I just got my first Exotic drop after 40 hours, had to buy my other piece of Exotic gear from Xur. Legendaries I've had a few of, sounds like you're just having awful luck.