I think it's because of a very pervasive myth in society that in order to lose weight, you absolutely must exercise those calories off. A lot of people don't seem to understand that their body does actually expend a set amount   3000  of energy daily that they can easily calculate online and use as a reference guide to allocate a specific amount of calories for the day in order to create a deficit, surplus or balance. I remember I used to think like that as well, and it did take quite a lot of soul searching on Youtube and google/wikipedia scouring to find out what a BMR and TDEE meant and what it could be used for.