Why do so many Christians claim to be former Atheists?

[Pat Mustard]

I think it's to try and gain credibility with atheists. Playing both sides.

No it's simply a ploy to gain cred with fundagelicals. Becuase as Cecilia has laid out above, these "former atheists" simply trot out how much they were of the fundagelical strawman of atheism rather than talking about anything an atheist would have experienced. Everything about their "former" experience is simply in the lens of opposition to fundagelism, therefore it is there to speak to the fundies.