Given, logically and statistically, life, imcluding 'intelligent' life has/does/will occur 'g-zillions' (smile) of times. The question then becomes, why no ongoing, demonistrative and repeatable, fully public contact with humans?
1) They can't. (light speed, hibernation / generation ships, etc)
2) They won't (hey, the ufos humans see are teen-aliens out joyriding - and violating the 'reservation' we humans are on - for our own protection).
3) They don't survive long enough to actually leave their 'home' solar system.
I go with number 3. In the greater scheme of things, is 'intelligence' REALLY a un- trump-able survival trait? Our history is replete with societies that soil their own nests to the point of extinction. Archeologists call the garbage pile near ancients settlements a 'midden.' Its a treasure trove to scientists who want to study, know, and understand what the long dead settlement's inhabitants were about. Study the experience of the humans who settled Easter Island. It was an isolated ecosystem. The intelligent humans used their intelligence to use, abuse, and finally destroy the ecology that was supporting them on the island. They turned Easter Island into Midden Island. They were smart, clever, people. Engineers and politicians. Their intelligence actually hastened their extinction.
Guess what? "Space Ship Earth" is ALSO an Island. And we, with all our uber -technology, and wisdom, and cleverness, still have not solved the problem of "the commons." ((IN a nutshell, any resource that is public, common to all, will be used and abused as quick as possible, by those who can, becuause if they leave it be, someone else will use and consume it.))
So, we, in the name of our respective religions and tribes, and corporations, and nations, are using our uber-technology to consume the 'common' resources of planet earth, in an ever escalating upward spiral. We will, however, turn planet earth into one giant midden. Easter island (they had a different name for the island, i am sure) inevitably became midden island. Planet earth will become the midden planet. The spiral will collapse. <game over> <no reload possible, insufficient resources > My best guess is this happens to all intelligent species.
They aren't 'visiting us, because they turn their planets into garbage dumps, and their planets ceast to be able to support them. <game over>
I hope I am wrong.