It's because they are ALIENS!
The first Avatar was an alien who came down to wipe out all humans but then happened to find this other human attractive. So then it gave birth to some human/alien hybrids who could bend like Oma and Shu. Eventually, these hybrids evolved into 4 separate hybrid families competing against each other with unique and awesome bending powers all united by the first alien who has managed to pass it's genome down each generation, explaining the reincarnation of the Avatar.
I tell you because the CIA won't let you know!
This was probably better in my dream about Avatar and alien robots. I'm losing sanity points in addition to OWL points here am I? Well don't blame it on me, it's the CIA's fault!
Edited 2 total times.