Because Academy Awards are for Hollywood movies which are products of an industry. Producer just hires one director and the director doesn't have much say during the project. This is why in most Hollywood commercial films you cannot even recognise the director by only watching the movie, as they don't have a voice, it's not their work and you won't remember his/her name after the movie. Of course there are some exceptions to this. For example: David Lynch who achieves to make auteur films, with his own stories and own style of telling them. But these people are rare in Hollywood. And of course these directors never get the attention of Oscars, because they are not commercial. Even Kubrick (he was more famous than Lynch) did not get a best film or director award from Academy and in America, it has become more and more about money. Below is a quote from Lynch, from one of his interviews:
"Unfortunately, my ideas are not what you'd call commercial, and money really drives the boat these days. So I don't know what my future is. I don't have a clue what I'm going to be able to do in the world of cinema."
In Europe, this is not how it happens. For example in Cannes Film Festival, the director receives the best film award and the best director award. Because in France and most of Europe, you have auteur directors and films some people like to call "art films". The director is the author of the film. It's his idea and vision. Without him/her, the film would not exist.
