why (...) NOT MAKE THE UPPER MID/TWEET DRIVER(S) ACTIVE AS WELL!!!?Good point. As you imply, it's a compromise of sorts.
The reason is that most of the energy goes in the bass region, so the biggest immediate improvement would be an amp dedicated to that region. As less energy hits the mid-highs, distortion is somewhat less AND more importantly, a good amp can control the system without being a mammoth and devoting +50% of its power to the bass alone (i.e. so much lost for the mid-high region).
I.e., as you go up the spectrum there's less musical info, so your average tweet rarely gets hit with more than 5W, and that's playing the system LOUD.