Oh, and another thing. You're also not lifting any of your body weight with the leg press machine, which you do in a squat.
Btw, that's something nobody realizes. Go ahead, put on a 30lb weight vest and squatting your 5RM will be much more difficult (even assuming that the vest doesn't get in the way or change your form at all). Leg press takes away stabilization needed, lifting at an angle and thus reducing force needed, plus takes away the bodyweight that you have to lift.
