Because it's easy to commodotise it. If all the designs are open then the race is to who can manufacture it better but there's a technological limit to that and the return on investment is quite low. What would be a company's market differentiator in this? Quality of construction is easily achievable nowadays, price will be low (as the market is very price-sensitive, who will be buying US$ 100 cases for their Pis?), there is not much incentive to participate in it, even more as the market is quite niche.

Maybe a company doing the packaging and selling of open-source hardware as whole product, much like as Mutable Instruments did with modular synthesizers, could work but then you aren't simply making cases anymore.

Chinese companies don't seem to mind doing this. The answer to thread OPs question is that all the stuff they want is sold by Chinese manufacturers.