It's not about motivation, it's about discipline. Motivation is fleeting, and if someone only exercises when motivated it's going to be hard to keep going. Discipline is what will get a person to exercise enough that it becomes part of their life. But to your point, discipline is not that common.

I've done some sort of workout almost every day for the last 25+ years. It had nothing to do with motivation, but making exercise a part of my day like brushing my teeth.