Because we don't see any of it. We don't see his network, his background, or his martial arts training, we just have to assume it exists. That's not background. One could certainly make a great background taking in how he developed his network and learned martial arts and grew to be the best in the business, but for all we see in the movie he could have sprung fully formed from the head of Zeus.

Ariadne's introduction as one of Miles' students is one reason why I placed her as slightly less flat than Arthur - the other being that the character has more reason to suddenly become entirely absorbed in Cobb and his world.