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.
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.