Squats not only train your legs to be stronger, they also strengthen your core. Sitting in the hole with weight at the front of your body can be nearly impossible if you have weak abdominals. Give yourself a good start by maintaining an erect torso and high elbows. Engage your core by taking a deep breath and pushing out and down with your inner abs.
Keep that tension in your abs as you squat. The tighter you can keep your abs, the less likely you'll fall forward and miss a lift.
We're working the chest and back more because they're a large part of your body. To maintain balance in strength and size, we need to train big muscle groups more than once per week.