An intriguing observation. How are you defining 'atheist,' then?
One who does not believe in the existence of any god.
My observation is that almost no one I have ever met, or read, or heard about, believes in the existence of any god.
Many people say things or pantomime their way through rituals in such a way as to appear to be theist as a survival instinct... but none of them have ever demonstrated a belief in existence of a god.
I will stand corrected if provided sufficient evidence of someone's belief, though.
If they say they believe, they've demonstrated their belief. You stand corrected if you cannot show that people don't believe what they say they believe.
What people say demonstrates only that they uttered some words. To take that as clear evidence of what they actually believe requires you to make several highly questionable assumptions:
1. That people reliably share your conception of what their words refer to.
2. That people are reliable sincere and honest and try to convey that shared meaning, and do not lie or attempt to utter the words b/c they serves a social function regardless of their actual meaning.
3. That they are able to accurately and reliably ascertain the contents of their own mental states and report it to others.
We know that #1 is invalid and that people not only use the word "god" to mean just about anything but even the word "believe" is not used reliably. Only if they mean "believe" in the sense of "I think that X is an objectively real thing that exists independently of any human ideas, hopes, and wishes." does it imply they are conveying that they are a theist. But "belief" is often used as a substitute for "hope", "wish", or a subjective state of preference for they type of world one desires. Complicating that is the fact that many if not most people are not able to even recognize the difference between those meanings, which is the crux of the whole is/ought problem in which people confuse what they think ought to be for what is and thus treat morality as though "X is immoral" conveys some objective fact about X" rather than the subjective preference (even if it's God's preference) that is neccessary for that statement to contain any meaning.
And in the context of God, we know there are massive motives for people to desire for their God to exist and that the is/ought distinction is constantly blurred in this context. So, this issue especially undermines any interpretation of what the statement "I believe in God." actually implies about their actual psychological state regarding the god concept.
We know that #2 is invalid b/c people lie constantly to themselves and others, especially about everything related to religion, and more generally about things where there are strong social pressures and sanctions to say particular things, which is more true of theism than just about anything.
Finally, we know that premise #3 is invalid, b/c there is lot's of research showing that people are terrible at accurately evaluating their own state of mind (aka metacognition). People cannot even remember what their own state of mind was 30 seconds ago, if that state of mind has changed. For example, you give people a scenario and ask them how they feel about it and what they think is going to happen next they will tell you one thing. But then after new stuff happens, you ask them to report how they felt and thought prior to that new stuff they are incapable of doing it. Instead, they think they always felt and thought the way they do now in response to the new information (aka, the hindsight bias).
Given these conditions must be met and we know they often are not met, additional evidence is required to infer that a person who utters the phrase "I believe in God." actually believes in a particular entity meets the definition criteria of God. And if you want to argue that it can mean whatever the believer wants, then the statement "I believe in God" doesn't mean anything at all b/c it could mean anything, including the opposite of all those anythings.