Even if that were the case, exercise can still be enjoyable for its own sake - and the high of having done it means the hours after are more enjoyable, too.
I have gone through multiple bouts of being able to do weightlifting for example for 6 months or so at a time. However, I always end up stopping (for longer than I could do it for) because it just isn't enjoyable. The monotony of doing the same lifts over and over again (even if the weight was increasing) just wasn't engaging enough to keep it as a part of my lifestyle.
I discovered rock climbing a year and a half ago and have been going every 2 or 3 days since (other than 2 weeks off one time due to injury). It's such an interesting sport that combines strength, agility, problem solving and collaboration. It's actually engaging which means I never have to be disciplined to continue doing it on days I don't feel up to it. The exercise isn't the point and yet my body is in better shape then it's ever been.
I think more people need to find that something.