The same reason why you'd bring a r i f l e to hunt rabbit instead of bringing one of these to hunt the rabbit:
With a harsher AI, more and harsher side effects come with it. I don't want to be pumping iron and my joints aching like hell. SOME estrogen is still required. Now, if I develop some gyno, THEN I will go ahead and use Letro. But otherwise, there is really no need. Arimidex and Aromasin apparently deactivate the aromatase enzyme enough to remove water retention and gyno. If it does the job, then there's no need to go further.