Why do atheists know more about religion then the actual religious?
Generally speaking, we do not. Arguably, we simply do not, without any qualifiers.
But it may well appear otherwise due to certain doctrinary and proselitistic policies.
The trick is in deciding who should be considered a religious person. My take on it is somewhat restrictive.
The random Christian or Muslim represents an entirely different situation. Those two doctrines spread largely by, for lack of a better expression, "presumption of belief". It has actually become commonplace to see Christian or Muslim parents speaking of their hypothetical children as if they had the power to establish their adherence before they even
exist. Cases of parents deciding to bury and deny outright the actual belief stances of their children are not even rare.
When it becomes an actual tradition to gloss over any indications of interest level, it is only natural for the average adherent to be uneducated about his or her own nominal doctrine.
You will have a considerably harder time seeking experts on Judaism, Paganism, Shinto, Sikhism, Hinduism or Jainism among atheists, because atheists or would-be atheists are not very often pressured into adhering to those doctrines. Judaism is something of a special case.
It makes more sense that the practitioner of a said religion would know more about it then an atheist. You learn more by hands on experience anyhow.
But also, why do atheists spend so much time learning religion in order to bash it IF they are so sure its all wrong?
Legitimate self-defense, buddy.
In that case, why dont they go spend there time enjoying there ONLY ONE LIFE they got. Why do they WASTE so much time studying religion?
It takes actual knowledge both to offer respectful rejection of what is thrown at us and to fiercely challenge it when necessary.