I asked my retired, optometrist wife.

She didn't have time to respond fully because she's dealing with a plumbing hardware supplier to get a defective toilet tank replaced*, but she sent this:

Those are for adults with presbyopia and near vision. The PD is standard for average adults. If we assume people will get the right distance prescription via over-the-counter means, then who is responsible if they buy the wrong thing and get into a car accident because they couldn't see at a distance?

I had to look it up, but "presbyopia and near vision" means you used to be able to see up close, but now you're old and you can't focus up close anymore. As opposed to: you're young, but your eyes are the wrong shape.

PD would be pupillary distance, ie the gap between your two pupils. One of the things they measure when they're ordering lenses for your glasses. As has been explained to me previously, if the PD is wrong, it's adding prism to the lenses, and headaches to your experience.

* She didn't retire to become a plumber. We're getting a powder room renovated, and the tank for the new toilet arrived damaged.

I'm addition to what was already mentioned, I will say that is not like that everywhere. In Europe I can go to an optician and they will do the eye test and sell you glasses without any kind of medical prescription. I would only see the actual eye doctor if I had some medical issue (e.g. diabetes).

I suspect some of that relates to reimbursement. My insurance in the US covers my eye glasses and checkups. In Europe my glasses were not covered.

The target audience for the gas-station reading glasses are people aged 45+ that need something good enough to read about 30 minutes per day.

If you are near sighed you'll probably use them all day every day.

The ophthalmologists in the room have spoken.

I call it bullshit. No ophthalmologist has ever gone to jail because some dude with myopia ran over two cats and a cute puppy dog. Usually, whoever is driving gets into trouble.

But let's think for a moment. If wearing glasses that are not your prescription actually affect your prescription in some way, then couldn't we figure out a way to reverse near-and far-sidedness? Like if you're near sighted and then use +10 (strong positive lenses, thicker in the center) maybe that makes your eye revert to better vision. And vise versa for far sighted people wearing -10 lenses for say 2 hours per day or something.

Instead of the bull, I would love to be able to find some glasses at the Walmart that I can use because I just lost my prescription or something. If there's a lens center position problem that too could be adjustable.

In short there's no reason. Its just how dumb things work.

Pupillary distance is more important for looking at a distance than looking at something close and eyes often have different prescriptions. Reading glasses are just magnifying glasses on your face.