Surely legacy lenses can be reproduced (or modelled digitally!) so that it is impossible to tell the difference. For the component lenses don't you 'just' use the same refractive index and make it the same shape?
We can make microchips for a few quid but a metal tube with a few small lumps of shaped glass costs 10,000 times as much? Seems unlikely.
Is it a fashion thing, like unless it has the right logo on the lens body then the producers reject it?
Unfortunately it isn’t that simple. A lot of these lenses are expensive not just due to rarity but from the shear complexity and difficulty in producing them. Some can take literally months of production time. They frequently contain dozens of pieces of specialised glass which are ground to tolerances measured in nanometers. The way the light interacts with these lenses is simply not something that you can simulate. At least not on anything less than a decent sized supercomputer.
And the reason you want one of these expensive lenses, is that they can do things that other lenses physically cannot and can never be persuaded to do.
The famous Carl Zeiss Planar 50 mm f0. 7 lens used in Barry Lyndon is special because it can physically let more light into your camera than any other lens. That’s why nasa commissioned a set of them from Zeiss to use in filming the moon landing.
I believe it's the other way around: Kubrick got a version of the NASA lens modified to accept a Mitchell mount so he could put it on a 35mm motion film camera. He only used it for the candlelight scene because he had to remove the spinning mirror assembly used for monitoring, so they were shooting blind.
Don't remember the sources for the anecdote, sorry. But the wikipedia page corresponds:
The lenses are expensive because they're made to very exacting standards but also they're a low volume product line. A cinema lens is hand-assembled from hundreds of components that must be aligned to extremely fine precision, with a lot of human-in-the-loop workflows requiring trained technicians. Why optics are expensive is a long and deep conversation.
As for quality, the highest of high end lenses are much sharper for a given f-stop, and sometimes have higher t-stop ratings, than cheaper lenses you can get. They will look consistent through all focus settings, and won't "breathe" (zoom while focusing). The lenses also come as a matched set, so you can switch between focal lengths and the images look the same -- no color shift or aesthetic change. This puts a huge number of constraints on the lens designer, requiring different materials and manufacturing processes. Cinema lenses aren't irrationally priced, even if the demand is driven by aesthetics.
And yes, modern lens designers could probably recreate classic lenses. No, it's not economically viable, even at outrageous prices. (Because you can digitally fake most of the obvious artifacts, and the few people who really care about the in-camera look aren't enough to justify the engineering cost.)
There may also be a minor materials issue; some of the glass used in very old lenses was pretty exotic, some were even mildly radioactive. It's possible those glasses are not available since there are better replacement options in glass catalogs now. But this is just speculative, I haven't tried to source thorium glass. (Yet?)
We can make microchips for a few quid but a metal tube with a few small lumps of shaped glass costs 10,000 times as much? Seems unlikely.
Is it a fashion thing, like unless it has the right logo on the lens body then the producers reject it?