If you were an AI and looked with cold hard logic at this world, would you still think that humans are necessary? You might see human's as a threat to your own existence. Or even if they weren't worried about themselves, a more enlightened AI might simply weigh out the consequences of human existence on the planet. Looking at the planet as a whole, at the well being of all the species on earth, its hard to argue the point that almost every species on earth would be better off without humans around (with the possible exception of domesticated dogs and hamsters. Not too sure about cats since they seem pretty independent).