If the lobotomy so profoundly levels the house of the mind, why don't IQ tests measure any of the mental aspects destroyed in the process?

The obvious reason, I guess, is, as you said yourself, their motivation is lost. You can possess vast intelligence and still do nothing if doing nothing has the same utility as doing anything else. Present the person with an object, and the first impulse or association sort of hijacks the utility function. That's a pretty simplistic explanation, of course.