One: We can't possibly fathom how many few worlds and in how many places these other beings can survive- some even being able to survive in places we couldn't ourselves- places we wouldn't normally even think of looking because its IMPOSSIBLE.
TWO: Humans have tendencies to lie. Why? Well... if you could tell everyone in the world that there were some aliens on mars... don't you think everyone would be like: HEY! WHAT IF THERE'S MORE!? We should go! Then we would swallow more than we could chew: Galactic Politics.
THREE: Whose to say galactic policies don't already exist? It wouldn't be the first time something surprising like that happened. If aliens with higher technology came to earth and bribed humans to keep them secret by sharing tech, allowing them to go on about the planet, making a sort of deal, wouldn't you keep it? (Especially if the alien could blow up your planet...) But even give you tech that could make you so powerful you feel like a god?
FOUR: Just when we say unicorns don't exist, you find frogs in geodes that have survived a thousand years within them... but they die when you return them to regular pond water. Google it. My internet isn't working. Look for the impossible, and you WILL find it, whether its good or bad.
I believe in extraterrestrial life, and intelligent life elsewhere in the cosmos. I do not believe we will ever make meaningful contact with that life. It would be awesome, but the odds are astronomically against.
There's a slight chance that they are, but I seriously doubt it. Scientists HAVE discovered new worlds out there, similar to ours, with their own "suns" and "moons" which I find highly fascinating. LOL If aliens are real then when they get to earth they will be more far advanced then we ever were.