My belief is that myopia is often caused by -- ach, let me be more circumspect. I believe that a contributing factor to most peoples' myopia is immobility. The muscles that control the eye are tiny; the tolerances of eye movement/focus are very very small. Therefore, any compromise to the ability of those muscles to properly execute tiny tiny movements has a dramatic effect on the ability to focus.
When we spend time indoors less movement of the eye is needed because the field we're viewing is smaller on all three dimensions. Training children to sit still makes it worse. Spending long hours reading or in front of a computer makes it worse. Our eye muscles stiffen from disuse.
I was diagnosed w/ myopia nearly 40 years ago. I quit wearing glasses for the most part about 30 years ago. But it wasn't until I learned to release the stiffness and tension from my eyes that I regained my clear vision. In my case that was partly due to releasing a habit of fear/anxiety that was inhibiting the natural movement of my eyes and neck. Frozen bunny syndrome on a micromuscular level. But the release was also partly physical. I learned to move my head and neck instead of holding them rigid.