It induces a hormetic effect on the eyeballs, causing them to grow longer over time, inducing blurriness. As you increase the strength of the prescription glasses, the hormetic effect will continue in a feedback loop, worsening your vision as your eyeball elongates.
It's the same adaptation that causes muscles to weaken, strengthen, bones to harden, calluses to form.
"Well then, if that's true, why was my vision blurry as a child when I wasn't wearing glasses?"
The initial blurriness one experiences as a child isn't caused by glasses and is likely a consequence of the ciliary muscle becoming "exhausted" from prolonged staring up close. Your eye muscles are actually most relaxed when staring at objects in the far distance. It evolved that way. We didn't "evolve" for close up work.
We know glasses induce myopia because we can reliably cause myopia in all sorts of animals by making them wear contact lenses. There's no controversy about this. Feel free to look up the studies yourself.
What's less plausible is whether the axial elongation can be reversed in humans. And it does seem (with some effort and time) it can be, although there's considerably pushback from poorly designed scientific studies suggesting otherwise but lots of Internet communities of people who have improved their vision significantly.
You can induce a reverse hormetic effect by wearing glasses that are on the "border" of blurriness and staring at objects in the distance for long periods of time. In other words, wear slightly weaker prescription glasses than what you're currently wearing; one for close-up work and one for far distance so you're always just on the "edge" of blurriness.
You can usually improve your vision at a rate of 0.75 diopters per year if you do this consistently.
At this point, I usually get a lot of disbelief and criticisms, links to randomized controlled studies suggesting the opposite. I get into arguments trying to explain how those studies were set up wrong, people call me crazy, etc. It's pretty annoying.
So I simply offer this: If you wear glasses, try ordering a slightly weaker version from Zenni Optical (or any store of your choosing). Try it for a year. See what happens.
It's cheap. And low risk.
"Don't I need a prescription?"
You do, in the sense that you need your prescription strength number, but the word "prescription" here is a misnomer. Anybody can order prescription glasses online legally. It's like ordering socks. It is not like ordering prescription drugs. Completely different legal classifications.
But if you're still skeptical, I ask you to think about this problem from first principles for a moment.
If Homo Sapiens started naturally going blind from the age of 9, our ancestors would not have survived long. Most animals that have poor vision usually compensate with some other sense. And while it may be possible humans just naturally evolved to have terrible vision, studies of other populations (like the Moken population that can see underwater and have near-perfect vision) suggest it's likely a modern problem.
There is a stressor. What is the stressor? It's multi-variate, but the most influential in modernity is wearing glasses.
I wear two pairs. One for close-up work (so it's slightly blurry when I'm programming). And another that's much stronger (and safer) for driving. So I can see all the stop signs clearly, but the mountains in the distance are just blurry. In this way, I'm always at the "edge" of blurriness.
There's a few other techniques you can do, but that one is the easiest to maintain. (Interestingly enough, people who hike outdoors a lot seem to do the best).
As you improve you'll find yourself ordering weaker and weaker prescriptions (although the closer you get to 0 diopters, the longer it seems to take).