IT is also logical for a species to secretly manipulate humanity to be more peaceful, reducing population, disabling nukes and other weapons especially (or making them controlled by one gov) and be more environment and resource efficient.
If I wanted to conquer Earth for its bio resources and location, invading with UFO's head on would result:
-Humanity retaliating with nukes from hundreds of locations. Even if aliens win, the result would cost them a lot and irradiate all the precious biosphere.
-Armed and numerous humanity will put up a fight and even when token fighting, will make exploitation more costly.
So if you are the leader of an alien invasion force, whats the best thing to do?
Infiltrate government, ruin target planet's independence and business, increase taxes, make costly environmental cleaning at the cost of many businesses, make strict gun controls, disable all the nukes, make people extremely dependent on Government.
Makes humanity an easy target. Initiate invasion.
Just a thought.