People don't know of a god until they learn of them. This makes them an agnostic atheist by default. My family stopped going to church before I could even remember us going to church. I never really understood the concept of religion before I made it into my teenage years. God was just a word to me. I know from experience that we do not start off believing. We start off questioning the world, which makes us susceptible to a religious answer, but we don't really think that God did something until somebody suggests it.
Humans, as infants, do not have the thinking capacity in their underdeveloped brains to contemplate the god question. This is why I'm an agnostic, because no one taught me otherwise. 99.9% of religious people are religious because their parents are. Christians are taught Christianity by their elders, and then the taught become the elders.
Humans are born new to everything. They don't have a deep sense of understanding about their surroundings let alone the ability to have a deep faith and understanding of higher beings or a denial of these things. If a child never hears the people around them speak of God or religion, then they will grow up not knowing anything about these things, rather than them having faith in God despite never having heard of God.
All humans are born agnostic in that they are born without the belief in gods or spirits. Only after they are told stories by their parents or community members do they start believing in the supernatural. No baby knows about the concept of god, so they are born agnostic. This is why Christians tend to make Christian babies and Muslims tend to make Muslim babies and so on.
Agnosticism is a person who believes that nothing is known or can be known of the existence or nature of God. However, babies are ignorant of their ignorance. They don't even know that they don't know. By that very definition, humans aren't born with any belief, so the lack thereof wouldn't be affirming the ideology. This perception is demonstrated in the "tabula rasa" or blank slate, epistemological theory.
Of course no one can ask an infant whether or not he believes in God, but all humans have an innate sense of a supreme being. All cultures believe in deities of some kind, and it part of our nature to understand there are forces bigger than themselves. Claiming to be an agnostic is simply the rationalization of someone not willing to admit what he knows is true.
In most cases ones religion is chosen by how they were raised but this is not always the case, some people can have outstanding Christian parents or be taught the bible and still turn out agnostic. Yes everyone is born atheist technically until that child is baptized and taught then they start there choice if whether they want to believe in a god if they choose. Sometimes a persons' way of thinking is also a factor in what they'll choose if someone is more of was logical thinker they are more likely to head towards being agnostic or atheist.