Because a) it's not as much based around a hatred of anything, more a worldview that refuses to resign the mysterious to the supernatural, b) because when you make the call that you must be atheist, given your world view, you are forced to question your position on morality and where you stand in society, humanity and indeed the universe. Nothing is hand waved away and you are now wholly responsible for your own definitions of these things, and the potential alienation that may result.

I can see how there is overlap between spirituality and atheism, as there's no need for a god or belief in most of the subjects that seem to fall under spirituality. Especially when we exist in a universe, the chances of which even existing are infinitesimally minute, that is so breathtaking that most of us have resorted to filling in the knowledge gaps of how and why with best guesses. It is so wondrous that, god or not, you can feel plenty spiritual just experiencing it.

Whether personal spirituality should be your life's focus when there's so much at play on Earth, however, is another question.