That's because no one produced any decent ones. Dave Wrath-Sharman made a handful to his own design for Cleland which were superb, reputed to be capable of working for a decade of daily use without attention, but there couldn't have been more than 2 dozen pairs ever made. That pretty much leaves Sachs, Shimano and Sturmey, all of whom made half arsed jobs that never really worked properly.
If DWS can design and build watertight, effective, durable and light brakes in his shed in Yorkshire, then why can't the might of Shimano do it in their post design labs?