That's not necessarily true. When I started, 135lbs was legitimately difficult. I wasn't strong enough, I wasn't coordinated enough. I hadn't learned how to squat right. That's half of what squatting multiple times per week is about - learning how to squat the best you can with good form. And I'm telling you that research doesn't support what you say, with regards to learning any technique. Frequency is key and the musculoskeletal system can handle to weights if you 1) start light initially, 2) only complete 3 sets of 5 at your goal weight. If you were doing a 10+ rep squat program, that changes a little bit, but the squat frequency is less of an issue due to the low volume in a given day.
And I agree with this for real first time beginners. Once the exercise is learned, it's time to up the weight and drop the frequency
I didn't want to get into an anecdote battle. Neurological gains are very, very important in learning a new movement, regardless of what it is. Frequency is key for neurological gains. Hence, squatting frequently is beneficial and, if you aren't feeling tired or like you're overrreaching, then your muscular system will be fine as well. It's great that you squat 400lbs, but there are guys that squat more than you who squat every single day (look at Olympic weightlifters or Dave Lipson's 365 days of squatting where he squats 400lbs+ every day for a year).
Good for them.. my work only allows me time to train for 4.5 months out of the year. Doubt many of those people would be at comparable strength levels as me with such a limited amount of training time
If you're in the gym 3x a week compared to 5-6x per week and you have similar gains, then I call that a more effective program. Considering Starting Strength includes upper body each day as well (a push and a pull), and you only have to do it 3x per week, you get massive overall gains with fewer bouts of gym-visits.