If this question means: Do I believe extra-terrestrials have visited our planet via inter-stellar flight? Yes. There are mounds and mounds of evidence to suggest that they have. My opinion is that they have had direct contact and have influenced our world as we know it today. (i.e. technology and architecture)
With the known size of the universe and the known age of the universe, it would be extremely narrow minded to say a) we're the only intelligent life in it, b) there is no civilization capable of inter-stellar flight.
As for the question of why haven't 'they' made worldwide contact...would you, if you were them? We're a planet of constant war, famine and self destruction. If I were an outside observer of Earth, I would recommend to avoid us at all costs. As humans, we cannot even agree to not attack each other based on one-world religions. Imagine if extra-terrestrials made contact, actually coming in peace? Our inability as a species to accept anything different nearly guarantees that they would be attacked on first sight.