Go around your house, or if you dont have any of these kinds of switches go to a hardware store, see how easy or hard it is to put and leave the switch in the middle of on an off, adding a third state, now try to see if you cant make for distinguished positions. Another example, take a coke can or beer bottle or any other object that is cylindrical and lay it on its side, then balance a marble on the top, how easy and fast and stable is that balanced marble?
using a transistor as a switch is very easy, drive it to one rail or the other, easy to sense the output. Now if you were to try to have all the transistors not be on off switches but instead calibrated to different ranges one for each state (in addition to all on and all off, two middle states as you suggest). Now the entire system has to be much more accurate, expensive, subject to error and failure, etc.
Basically this was tried, an or some early computers tried to be decimal (10 voltage levels), it failed. be it a tube transistor or silicon, it is significantly easier, cheaper, faster, more reliable to use the transistor as a switch and have only two states, lower rail and upper rail.