My personal squat form is not perfect, and neither is anybody else's who is not constantly coached. I know what is supposed to look like, but because I can't see it while I'm doing it, my form will drift over several workouts from good to bad. It will settle into a stable, bad technique and will remain there until I get it corrected again. This is called "form creep", and affects all uncoached athletes to varying degrees.
Everybody needs a coach, no matter how accomplished they are. A coach provides a good set of eyes and the experience to know what to say to get you to do a movement correctly. This is the primary problem with training alone at home, not the lack of some asshole yelling that it's "All you, man!" You can't see it as it's being done, and you can't correct it as it happens if you can't tell what's wrong. Video is useful, but it's no substitute for a coach's correction in real time, your feeling the correction and incorporating it into the movement pattern, and then your feeling the difference the correction made from wrong to right during the set.