Because it's not "atheist" I said was the problem, and because "don't know" isn't my default state. I am pretty damn sure there's no eye-in-the-sky watching over us. What I am, is "willing to accept that I may be wrong, if appropriate evidence presents itself." Agnostic says "we don't know" and sometimes "we can't know". I don't feel either way on that. Like I said I am pretty damn sure one direction, but it's not "being rational" to cling to a theory that later gets disproven.
But what I don't feel the need to do is point my finger and laugh at those who do believe in something. Yet, go into any public forum where there is a post on religion, and you will find a dogpile of people doing exactly that, while claiming how superior their interpretation is. And I use "interpretation", because that is what science does with data to get results. And that is the kind of behaviour I have a problem with. My interpretation of the data comes out the same: there is no evidence pointing to a higher power, but I don't feel the need to be smug or judgemental about it. The history of science is riddled with those who couldn't readjust their thinking in the face of new discoveries and got left behind.
Short version: keeping an open mind if new data emerges and can be tested isn't saying "we don't / can't know". But like an astronomer who doesn't believe in the existence of little green men on Mars, it's important to be able to also examine that and maybe revise the theory if one walked over to Curiosity's camera and waved.