You have just never tried to find God.
If atheists' nonbelief is based on a rational, sober assessment of the evidence, reaching the conclusion that belief in gods is unreasonable, this challenges a religion's claims to truths as well as the assumption that theism is rational. Not all adherents of theistic religions recognize the significance of the challenge posed by the mere existence of atheists, but many do and this may why they have tried to come up with excuses for why atheists don't believe. Here we have a popular one: the allegation that, unlike theists, the atheists never really tried in the first place.
Unless the above assertion occurs in a discussion where the people know each other very well, it can't help but give the impression that the speaker is incredibly arrogant. How on earth can a theist (usually a Christian, as it happens) who doesn't know me claim that I "never really tried"? I don't think it's a coincidence that such theists don't hesitate to make a host of other arrogant assumptions about me or about whatever atheist they are having a conversation with.
It's as if they aren't interested in talking to a real atheist, but rather to some amalgam of the prejudices and assumptions they've collected over time from reading books by apologists. I don't think any of the popular apologetics books written for Christians about atheists isn't completely undermined by repeating one myth and mistaken assumption after another and this seems to be a popular one.
The fact of the matter is, many atheists you will meet were once very religious indeed, a majority were probably Christians of one sort or another. As such, they did try very hard to find and understand God. Many will tell you that they trusted in Jesus to take over their lives and that they prayed the prayer about Jesus entering their hearts and were born again.
Did they mean it? As much as any other Christian. People commit the fallacy of Special Pleading if they try to claim that an atheist couldn't have "really" meant it if they aren't a Christian now. They often attended Bible studies, both as a child and/or as an adult. They often had discussions long into the night with a Vicar, Pastor, or Elder. They prayed, and were prayed for. They attended revivalist meetings and sang their hearts out. They studied and asked for signs. They worked very hard trying to find God.
In the end, though, neither God nor religion "worked." Doubts appeared and questions grew more intense and more difficult. Satisfactory answers did not appear, either from religious leaders or from God. Either He didn't want to help, or He wasn't there in fact, this lack of contact and persistence of nonbelief among people is itself a very good reason to doubt the existence of many alleged gods.
Some theists will then go on to assert: You weren't really trying in your heart.
But how do they know? I know that I was, as hard as I could. They don't think that I was just deliberately wasting my time, do they? This second claim is more arrogant than the first one above because no one can so confidently claim to know the "heart" of another person when they have never met or did not know each other during the time in question, much less today.