Just another speculation. Cause it's fun.

Premises: Rather benign aliens with psychology somewhat similar to our own.

If UFO:s really are about alien spaceships, and if I were an Alien onboard, I would be careful with that first contact with humans.

Probably General Ezxoophyth and his superiors have ordered all ships to remain in stealth mode for the time being.

I think they realize that humans will react in may different ways, some might be enthusiastic, other openly hostile (and maybe dangerous, even to little grey bodies) others will be covert enemies, organising underground, planning to use heavy weapons against the "foreigners". They know that humans will tend to be very suspicious, feel very unsafe and insecure, and thay might have had the same experiences as we have from disastrous meetings between very different cultures.

In other words: If they announce themselves very clear and unambiguosly - some kind of hell will break loose and that would be dangerous and contraproductive even for them.

Of course, they have fast vessels and maybe fantastic weapons, but what's the point of getting into a war with humans? If they'd be forced to kill one human, even in self-defense, humankind would never ever trust them and further contact will be almost impossible.

Maybe they, the aliens in their ships, show themselves once in a while, just to test us and/or to make us accustomed to the idea. Like a psycho-cultural priming.