They did not evolve in areas where the only way to get warm enough to keep their metabolism functioning approrpriately was to plaster themselves to a superheated rock, basking light, or heating pad. Unlike humans, who developed reflexes to move themselves rapidly away from superheated heat sources, this rapid withdraw reflex wasn't needed by wild reptiles. Humans, with their more complex brain structures, are able to learn to do this. While many species of reptiles show signs of learning things in captivity, it is clear from the thousands of reptiles presented each year to vets with life-threatening thermal burns that reptiles can't learn this. This does not mean that they are not in pain.
Humans can and do mask pain.
Some may be so used to a certain level of pain that they adapt to it in the sense that their body appears to continue to function as usual, or with some accomodation, but this ignores the metabolic and hormonal processes going on inside the body which conntiue to exact a high price for the body continuing to have pain. Atheletes may mask pain in order to keep from being benched or getting a lower bid on their contracts. Parents and teachers may mask pain to prevent their children or students from worrying about them. People with invisible disabilities , which may include severe muscle and joint pain, are often hassled when they park in disabled parking spaces, despite the fact that they have the proper permit, obtained by their physician completing the necessary forms to get the permit. They may groan, or grunt, or use spoken or written words to describe where and how much it hurts.