This doesn't work as an explanation, lenses are always cut to the frame.

My brother in law owned a lens cutting shop and they have boxes filled with lenses ground to all sorts of prescriptions that are circular plugs 3 or 4 inches in diameter. When the order comes in they use the pupil measurements and the frames to cut the lenses for the prescription down to fit into the frame.

I don't know how well I explained that but you can watch this video to see exactly how it works: https://www.youtube.com/watch?v=uCjGNUPO0WU

Thanks for this explanation - makes complete sense but never thought about it!

Nah. Outside of the vision cartel you can get replacement lenses for any pair of frames < $100, with all the various coatings and special shit for low hundreds. Likewise, using insurance to just replace the lenses, if a plan includes that, is often $50 or less. It's just shopping at the vision cartel without insurance = huge sticker price.

Any tips for how to stay "outside the vision cartel"? (In the US, if that is relevant...)

zenni optical. I mostly use contacts, but I have several 'backup' glasses. You can get decent looks frame for like $11 shipped.

Yeah; that won't work for replacing lenses in existing frames, but for a new pair it's what I do. $50-60 and I've got my prescription in a frame I like with all the coatings.