It's not just definitions, it's also that a lot of people experience actual adverse reactions to exercise [1]. The typical social script is to condescendingly explain to them how they must be doing it wrong, and doctors often have trouble finding a medical explanation (even when it actually is caused by a medical condition), so people often blame themselves for having problems. This is, to put it mildly, not an optimal experience.
https://en.wikipedia.org/wiki/Exercise_intolerance