They are pretty common in places where people actually do need to type quickly(stenotypes are what, 1890s to present day for court reporters?); but there don’t seem to be that many applications where very, very, high speed is considered worth the expertise required to operate the system and the loss of all the various fiddly symbols that have gained assorted domain-specific uses.
It just appears that places where you need to type very quickly are comparatively rare. You can usually type faster than you can think, with a little practice, on an ordinary keyboard; and ‘can be implemented in software’ appears to be the highest law for keyboards serving sub-laptop devices.
