The other thing about heavy squatting that other "leg exercises" don't give is the total body stimulus. What I mean by that is the sheer volume of muscle mass that works during a heavy squat. Quads sure, but what about glutes, hamstrings, calves, adductors, abductors, lower back, upper back, abdominals, hell even shoulders and traps get serious stimulation supporting a heavy bar while maintaining tightness.
YOU CAN GET A GREAT PHYSIQUE JUST FROM SQUATTING (including upper body)