How do people back squat heavy without hurting their upper spine?

Whenever I back squat over 100kg (220lbs) I need some sort of protection between the barbell and my body (the back of my neck), otherwise the weight just hurts where it's resting. I wrap my towel around the bar to help, I also used a specific protection in the past.

What baffles me is that I routinely see people squatting 400lbs+ with no protection, almost all of it resting directly on the spine, and it's never brought up as an issue. Do they become desensitized? Am I just doing it wrong?