When you kind of analyze it the squat has the resistance that first goes through you spine and lower back before it hits your legs. It's passing through a very unstable structure that's design more for mobility (twisting and turning). When the resistance is on your feet it is going through and being supported by the biggest, sturdiest bones in your body design for stability and is much more solid and doesn't twist and bend like you spine and lower back does.
People can do leg pressing movements well into old age and until they become a cripple. I rarely, actually have never seen, an older person, say 60+, still doing full squats.
We all start out doing squats but it takes it's toll and eventually everyone has to give it up.
I am a big believer in body weight squats and always do them right after a set of a leg press movement. Being able to do a full body weight squats is a very valuable and functional movement. Resting a heavy bar on your spine not so much.