Hollywood is very fond of creating aliens that come to Earth in order to conquer it and exploit its resources. It can be a highly entertaining view of extraterrestrials and feeds on the natural fears of humans, but it is just not logical.
Assuming that humans are not the most intelligent life form in the universe, then it makes sense that extraterrestrial beings have been watching this planet since the creation of nuclear energy. Having a nuclear capability gives man the power to destroy the entire planet. If this happens, then the hole created where Earth once lived would destabilize the orbits of every other planet in the solar system. The meteors and comets created by the explosion could possibly slam into any other star or planet in its path. All this destabilization would then spread out into the galaxy and could, in time, completely change the entire universe. This kind of power in the hands of the war-mongering human is very dangerous so, of course, extraterrestrials are watching humans closely.
If extraterrestrials had any desire to overtake humanity to control the planet ,they would have done so before the society reached this stage of technological advancement. Some argue that extraterrestrials already did this and are identified in the mythologies as gods and goddesses. These were, by definition, beings of a higher intelligence and introduced humans to things such as fire, iron, and alcohol. All the things needed to create conflict among earth's inhabitants. The proponents of this theory suggest that those gods and goddesses used the masses as slaves and used those humans they could trust as the managers of those slaves. Although the alien species has long ago left the planet there are still humans who think that the exploitation of the planet, as well as the beings living upon her, is the right way to behave.
Whether or not any of that is true, it does show that early humans would have been much easier to exploit than those of today. An aggressive invasion by an extraterrestrial species would cause an insurgency like the world has never seen. Forget car bombs. The military of every nation on Earth would unite and use lasers, sonars, and even nuclear weapons to ensure that another species did not gain control. The resulting destruction would not be in the invader's best interest. And, no matter how powerful the invader might be, there would always be humans who would never accept slavery and revolution would always be ready to break out.
It's much more likely that the extraterrestrial species watching this planet are of a more peaceful nature simply because they are of a more advanced technological stage than man. If they continued to be warlike they would have destroyed themselves before reaching the point in which they can travel time and space and remain relatively unseen in our skies.
Mankind is at that critical stage today. We can either continue our war-like ways and, in the end, destroy this planet and everything on it. Or we can learn to use our technology for the benefit of all of mankind. If we make the necessary steps toward becoming a more peaceful species, then there is the chance that those watching us will uncloak themselves and possibly share with us the technologies that can help us eliminate poverty and disease.
Hollywood likes to portray extraterrestrials as conquerors because that's how humans behave. Storytellers know that using the qualities humans are more familiar with sells well. If they made a movie in which aliens landed on this planet and demanded a world-wide nuclear disarmament and a permanent peace treaty people just wouldn't buy it. There just wouldn't be enough action and conflict. It doesn't matter that the peaceful scenario is more likely to happen than a bloody conquest.