The manual for a human body says several hours of exercise per day.
Children try to follow it, but it's being made hard for them, and by the time you're an adult most have learned to forget the natural instincts for movement and how much fun you can have doing physical exercise.
There is a theory that testosterone makes physical exercise fun. No idea if it's true, or to what extent.
Anyway it's completely possible you just didn't find the right physical activity yet. It took me some years to realize that I have to exercise alone and outdoors.
It depends on what you're doing. I could play sports for hours and completely lose my sense of time. If I had to run on a treadmill for 20 minutes I'd die of boredom.
Children try to follow it, but it's being made hard for them, and by the time you're an adult most have learned to forget the natural instincts for movement and how much fun you can have doing physical exercise.