That's because those $20 glasses are objectively worse than $200 Costco glasses. You might not think that the latter is worth the increased price, but for the vast majority of Americans it is.
Btw Zeiss (known to HNers for making EUV lithography hardware) also makes glasses and you can buy them online. Some of their lenses are $700. Not worth it for me, but I can definitely see how someone would be willing to pay even more for those optical specs
I have zeiss lenses on my glasses, they’re terrible. Full of scratches in 1.5 years. Worst money I’ve ever spent, my last no name €80 lenses went for years without a single scratch! Anecdata obviously
What, do you think they scratched them on purpose? Avoiding scratching for thousands of hours isn't actually simple, and if some brand is less scratchable then that's a real benefit.
The $30-50 zenni glasses/lenses in my experience are actually substantially clearer than the (absolute ripoff) $400-500 rayban ones. Same prescription, and the zenni ones even arrived quicker! Frame consistency is a bit of an issue though, I bought a few more cheap pairs with the 'same' frame a few years later and they changed the design subtly which made them fit worse.
Btw Zeiss (known to HNers for making EUV lithography hardware) also makes glasses and you can buy them online. Some of their lenses are $700. Not worth it for me, but I can definitely see how someone would be willing to pay even more for those optical specs