I admit, when I started running some eight-or-so years ago, my first smartphone with a GPS-tracker was a strong factor in keeping the motivation up. The feeling of "living the future" was fun, and measuring exactly how far and how fast I was going made progress easily visible.
In the meantime I've "logged" most of my exercise, be it runs, hikes, cycling, or skating on Swedish winter lakes. But I never got into wearing a heart-rate-monitor, or tracking my weight more than twice a year. Plus, I rarely checked the long-term statistics of my exercises - after all I had reached a level of fitness that I am satisfied with and lack the ambition to reach any lofty goals.
So instead of going along with the "quantified-self" movement, wearing a step-counter with unobtrusive heart-rate monitor, or tracking my diet, I recently found myself not even bothering to turn on the tracker-app when I bike 20km to work, and back. Running without a GPS-watch lets you pick up some mushrooms and look around for more, without the nagging feeling that your average speed statistic will be messed up by this. I generally prefer not being in a hurry and physical effort does not need to come with feeling rushed.
Then there is the whole data privacy aspect. Do I want everybody to exercise more and be healthier? Sure. Do I want insurance companies to punish people who are not willing to prove their exercise by sending them their "quantified self" data? I think not.
Whereever data accumulates, it creates desires from companies and governments to use it for originally unintended purposes.