I have to chime in on this.... my sister actually brought up this point (the OP's) and my response was....

Amanda believes that she is in good with Parvati and Cirie. They have their threesome going on and she would rather risk trusting them then going up against those three guys in immunities later down the line. With Jason, Erik, and James... Amanda doesn't have much of a shot of winning in challenges. With the girls however... she knows that she is the strongest and has the best odds to win immunity while keeping Parvati and Cirie close.

Now why didn't James, Erik, and/or Jason think of pulling together as a foursome? Simple... they are all fucking MORONS.