Leg presses dont buckle you over. And the angle is hard on squat compared to legg press. I find no carry over in strenght from one to the other. I squat mid 700's and can leg press a full loaded leg press with 45's hanging off the handles and a girl sitting on top holding a 45 i think not counting th egirl it was 28 plates full and deep for 6 to 8. And could do the same when i squatted 100lbs less then i do know.
Part of it is you don't have to stabilize a bar with your torso. Also the angle on the leg press decreases the weight so you aren't actually lifting all of the weight on the machine.
The squat is a total body, core exercise. It taxes the whole skeletal\muscle system. The leg press is more of an isolation exercise for the quads. The reason you see more people doing leg presses is because its easier. Oh yeah, I almost forgot, they don't want to get their hips too big either. BTW, some people with legit lower back problems will do these and I will not flame them for it.