All that fear, guilt, paranoia, awe, the what-if, the anxiety, the 'safety', your entire worldview, your self-identity, your way of life - it's not something so easily overcome with just pure reason and logic. Even when you know beyond doubt that Islam is bullshit, you might try to ignore it because Islam is your comfort zone, you'll keep following it to feel 'safe' - it's such a huge leap for many people to imagine living without Islam in their life.
In short, our emotions get in the way.
In short, this is ultimately it-leaving Islam, moving away from what you've been taught to believe is the 'truth' and then looking for meaning in other forms.