If there's so many UFO's here on earth why haven't the aliens made contact?

Because all those ufos are nothing more than Earthly phenomena such as airplanes, balloons, clouds, insects, mirages, or electrical discharges. The remainder are hoaxes, frauds or delusions.

Space boogiemen are totally unnecessary to explain "ufos". The true explanation is to be found in the natural sciences and in the psychology profession.

Because the aliens are superior

If they were in trouble, they would stop their ship and disembark and ask for help ; which may actually've happened

The aliens're travelling, exploring and observing. It's EASY for them to visit ; they're doing a job ; testing their technology against humans' ; etc.

UFO doesn't necessarily mean they're aliens... it just means it was something someone couldn't recognize.

Because you do not know what is a UFO. Those three letters are an abbreviation of 'unidentified flying object'. That abbreviation is not a word and is not a synonym of alien spacecraft.

Unidentified flying objects (UFOs) are objects that appear to the observer to be above them, vaguely in the sky (flying), but which at the time they are observed the observer could not identify the object (unidentified).

They are objects from, linked with or seen from Earth. They may be natural phenomena such as an astronomical object, e.g., a planet or star, cloud formation, flock of birds flying at high altitude. Alternatively, they may be man made objects such as artificial satellites, aircraft, weather balloons, etc. Whilst unidentifiable at the time to that particular observer, they are all subject to rational explanation.

What they are not is spacecraft carrying a species from another planet. There is not one iota of evidence that such has visited Earth. Plus when taking into account the manifold practical difficulties and the many dangers involved in travelling through space over huge distances, the probability of such visitations is very close to zero.

Maybe they look like trees and they try to communicate with us by swaying in the wind. 

Many will tell you that they have.