If reptiles in general, and snakes in particular, could live at a constant temperature, then there would be no such things as thermoregulation no terms called basking or "heat conservation coil" etc etc. There would be no behavioural mechanisms to achieve certain temps, higher or lower. Reptiles can ONLY change their temps by behavioural methods (being ectothermic), so it makes sense to provide a gradient. They use different temperatures to achieve different life events. To suggest that they only use one constant temperature is just ludicrous. You say that yours can be kept at 84F and they seem just fine. That's your observation (right or wrong) and can no way be applied to all living snakes. I can keep a TON of things ALIVE at 84F. Doesn't mean they are thriving or recruiting or completing life events. People kept monitors alive for decades at 84F, but they never bred. As soon as someone decided to give them 130F+ basking spots, they ate and bred like flies. Wonder why?
Keeping thing alive and observing them to NOT move when your cage is a certain temperature is NOT a measure of thriving. Its just a testimonial as to how hardy these damn things are, and how they can survive under such adverse conditions.