Why would we think that? Tolerating discomfort seems to be a skill that anyone can develop with deliberate practice. Perhaps genetics imposes some upper limit, but just like in sports most people never put in the work necessary to even approach their genetic limit.

https://eastermichael.com/book/

It's winter in the Northern hemisphere right now. Try going for a walk tomorrow deliberately undressed to the point that you're deeply uncomfortable but not risking serious injury or death. Anyone can do this, and over time it makes tolerating other forms of discomfort easier.

Metabolism also isn't immutable. Ask anyone who became vegetarian for example.