Perhaps we define stress differently. How can burnout happen if you love every little second of your day and everything you do fills you with joy and energy?
If you or anyone you know feels like that, then it's a serious outlier condition worthy of academic study.
Why not? Let's bet a few people do. There are certainly people who live their life in a permanent joyous state, no matter what. Rare, perhaps but I wouldn't dismiss their experience as a condition.
It can be boring, mundane, unfulfilling, uninteresting, repetitive, etc... That doesn't generally produce a traditional "stress" but does wear on you emotionally.
In fact jobs I loved and cared about have often brought much more stress exactly because I cared about the outcome, and when I failed to live up to my expectations at that job it felt like a personal failing and I was letting everyone down.