The problem is that they don't scale, same reason a 1.5l F1 v6 can run 15,000 rpm and a 5 liter american V8 can't. In order to support the forces the components would have to get bigger and heavier and eventually it's a point of diminishing returns.
Also the funny thing about horsepower, if you are making 1 hp at 20,000 rpm, you are making .2 lb-ft of torque, by the time you convert that to anything useable in human scale, the gears or whatever method you use to convert it adds so much drag and inertia it goes to nothing