If more individuals today are religious skeptics than in centuries past, that is mainly because accumulated knowledge has inclined more people toward such doubt.This is definitely true for me. As long as I was a little country gal living in the shadow of my ignorant parents I was a believer. I didn't know the difference. I mean no disrespect to my parents as they didn't know any different either; they followed their parents.
Interestingly, we can see that in many ways believers don't really choose either, but when we consider theistic beliefs we see different causal environmental factors at work. Early childhood indoctrination by family, for example, is a key environmental factor that promotes such beliefs in many, as is the pro-religion conditioning that one receives from the community and broader society. Even if the overt promotion of religiosity by society is mild (which usually isn't the case in much of America), prevailing social views that disapprove of open disbelief will often discourage serious exploration of secularity.
Since I have become active in the atheist community I am surprised at my fellow citizens who will deny in public their disbelief. I have a friend, a member of a social group I belong to who still goes to church on Sunday because she doesn't want her children to know that she doesn't believe. There so much pressure to conform in scociety.
I was thrilled when Mayor Blumberg refused to allow the 9/11 observence to become a religious service. There are so many of us who are patriotic citizens of this country but who do not believe in prayer to an unseeable god. To exclude us from society is unfair and unAmerican.