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?)