Don't have to have a reason to lack a belief, but I have a reason and it's fun to use on those that do believe.
If aliens really were here we wouldn't have to know it, for reasons I'll outline below.
We wouldn't have to know alien craft are here because they could hide. We humans, which can't even get out of our own system yet(in person), have aircraft that are very hard to see on RADAR. We're working on active optical camo(think Predator-like), could hit the battlefield by 2020, or sooner. A folded fabric suit that masks IR detection already exists. So, wouldn't a civilisation that's thousands of years, or at least hundreds, ahead of us have the technology to not be seen? Or, their craft could be the size of golfballs, and upon landing they make robots that look just like Earth life(unless you put them under a microscope). That fly on your wall—you get the idea. That tree in the jungle—laser transmitter to orbit, or farther. We'd be more clueless that they were here than targets are to a sniper wearing a ghillie suit. UFOs are not aliens, unless they just get kicks out of showing themselves and messing with people's heads.(talk about a pathetic rationalization) Aliens, until proven otherwise, aren't here. Though, like I've outlined already, they could be. Feel free to use this argument against UFOs-are-aliens believers.

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If we don't play god, who will?-James Watson
I never think of the future, it comes soon enough.-Albert Einstein
The large print giveth and the small print taketh away.-Tom Waits
Any sufficiently advanced technology is indistinguishable from a yo-yo.-Enoch Root, The Confusion
When I was a kid, if someone brandished a shrink gun he'd get a little bit of respect!-Myron Reducto, Harvey Birdman