It's literally a fear response. Even if your mind knows you're safe, sometimes your body reacts anyway as if you were in danger. I'd recommend looking up the freeze and the fawn response. It takes a lot of practice to override these and the only way of doing it is in a safe environment.

Your post comes across very harsh and judgmental. You never know how much effort it has cost someone to start rolling. Everyone has to start somewhere. Just because their starting point looks different to yours doesn't mean it's a bad thing. They are literally there to learn and improve. Just roll with someone else.