They're not intentionally letting their knees caved, that's a position of less power generation, but it can be useful for force acceptance and deceleration (eccentric) before setup for the push.
It's just the difference between ideal and max effort. I'm sure if you can complete a 200kg squat then watching yourself do 100kg or 200kg you'd find drastic form differences even though you're trying to maintain form. Bicep curls to deadlifts it's the same.
Dave Tate has some succinct but great quote about it that I can't remember. Essentially though it should be about protecting yourself and being able to lift continuously for progress and then from time to time you have ball-to-the-wall lifts (like the olympics) where it's only about making the lift no matter what. For these, for me, I'm maintaining form so I can lift next week... if I've trained for four years to be on stage for an hour or two and get a medal... I'm not going to worry too much about my knees caving a little
Also ... I wouldn't try and mimic world-class athletes too much, they have flexibility, strength, coaches, physio and everything else that most of us can only ever dream of. Protect yourself and train week in, week out, and you'll see progress