Literally, prescribe means before (pre) and write (scribe). It is
simply putting the numbers down on paper.

Why does it have to come from a licensed practitioner? Because there
are laws that say such. The laws were enacted as professions became
licensed and responsible for their actions, as well to protect the
professions. The wrong glasses can cause problems with depth
perception and acuity, thus causing drivers to crash and injure
people. They can cause eyestrain, double vision and headache.

You can also read posts from Cletis in this NG which is probably the
best reason not to allow open slather prescribing. The government
decided to protect the public from themselves and deluded individuals
like Cletis.

dr grant

I can see why prescriptions are required for drugs, because those can
be easily abused or sold in the black market. But to me, eye
prescriptions don't seem that critical.

My analogy is shoes. You could probably mess up your
feet/knees/legs/back by wearing the wrong size or type of shoes, or
improper inserts long term. Yet a shoe prescription from a podiatrist
is not required for people to buy shoes.

Of course, everyone should get an initial refraction done
professionally. But what's wrong with buying disposable contacts that
are a little higher or lower than your prescription to see how they
perform? You're not going to be seriously injured. Maybe get some
headaches or eyestrain, that's all. And if you do have problems, then
go see the optometrist (just like you'd see a podiatrist if shoes were
giving you problems).