Because people who DONT PRAY, because they don't have imaginary friends and don't believe such activity serves any purpose, would still call their meeting a 'prayer group' and pray to nothing. WTF?
On the other hand THEISTS who do pray to nothing hold meetings to reinforce their delusional pretense and pretend that what they are doing is meaningful because they fear NOT believing and that is in turn because they are taught their 'belief' is a moral imperative and that beliefs can be willfully decided, as if reality conforms to what they DESPERATETY WANT to be true... because of their commitment to the moral imperative and the, motive induced by induced by emotional blackmail and bribery of heaven and hell.
Pretending that atheists would go to a 'prayer meeting', is just moronic projection of the stupid things theists ACTUALY DO.