why don't the ranges for the RGB values go to 4096 (or whatever the right number is)?

Actually, the high bit depth number is 0-32768 for a total of 32,769 tones/channel when in "16 bit"...which isn't actually 16 bit but 15 bit plus one level. Why? You would need to ask the Photoshop engineers for an exact reason but the gist is there are no "true 16 bit" devices only 14 bit plus and 15 bit plus one level allows for an exact middle level which is apparently useful for image processing algorithms while still preserving the high bit depth precision...