tl;dr: It's not important to their life, so they haven't given it much consideration. (Someone can argue) atheism and agnosticism aren't exclusive, so your question is based on a faulty premise, so it is redundant. The lack of evidence for the existence of God might be counted as evidence for the non-existence of God. And people might be often pigeonholed into an answer by the way someone could frame the question.
I suppose you can just be an agnostic about anything. It just wouldn't make much of a semantic difference that affects your life a lot.
I consider myself an agnostic atheist; in that (agnostic) it's not possible whether to know whether a God exists or not. (It's also possible to be an agnostic theist, so atheism and agnosticism might not be mutually exclusive.)
Also, even if I was to give 50/50 equal consideration to the existence and the non-existence of a God before looking at the evidence, I can endlessly search for evidence and not come up with any (nothing that I consider evidence.) Someone might say you might not have just found the evidence yet, but if I see no visible signs [to me] in the world; if I cannot deduce anything, then I might say the lack of evidence is actually evidence of the absense of a God, so I'm now (slightly) in favour of the non-existence of God (like 40/60), so if I had to come to a conclusion (which people don't have to, but perhaps often feel the need to/it's nice to), I'd say God doesn't exist.
Perhaps more people will be of an agnostic viewpoint if the question wasn't often presented as "do you believe in God?"; which can pigeonhole you into "yes" or "no" instead of being given an open-ended question giving someone a reason to think about it more.
(On a side note it's an entirely different ball game then if someone believes whether they should let it affect their lives, but people often bring that up in these debates anyway.)