Why doesn't someone just make a cable actuated hydraulic brake?
Because you'd still have a cable, which is the one thing you're trying to eliminate for the sake of all-weather reliability. I reckon Shimano will be first with a fully hydraulic cyclo-cross disc set up, simply because they have the space to stuff a master cylinder into the body of a Di2 lever now that the shifter guts have been taken out. Rich kids and Shimano sponsored Pro CXers only, then, since that will be a big premium over the already eye-wateringly expensive Di2.
Because you'd still have a cable, which is the one thing you're trying to eliminate for the sake of all-weather reliability. I reckon Shimano will be first with a fully hydraulic cyclo-cross disc set up, simply because they have the space to stuff a master cylinder into the body of a Di2 lever now that the shifter guts have been taken out. Rich kids and Shimano sponsored Pro CXers only, then, since that will be a big premium over the already eye-wateringly expensive Di2.