There is more to this question than simply the presence of Migs in the IAF. The Migs trace their lineage to a diametrically opposed set of strategic and tactical considerations. The Soviet era Migs, to which the current derivatives owe their existence were meant to be easily replaceable and used in large numbers, with a general disregard for the pilot. This tied in to the general Soviet doctrine of trying to overwhelm Nato with sheer quantity. For this reason, the current Russian fighters are still playing catch in terms of man-machine interface, reliability, and fall short technologically in comparison to their contemporaries.
Beyond these considerations, induction of an aircraft isn't just a matter of placing these planes at Pakistani airbases and by default, have them dominate the skies on behalf of the PAF. A new platform requires extensive training for both pilots and maintenance staff, as well as a rethink of the tactical plans based on the strengths and limitations of the platform in question. A Russian aircraft, due to its history is a vastly different aircraft than the predominantly western and westernized Chinese aircraft PAF has historically operated.
American f-16s, Chinese jf-17s, and Russian Migs, all with a different set of weapons, maintenance procedures, and parts would be a nightmare for any air force. For a cash strapped one, it would be utter disaster. It is cheaper and more time efficient to streamline, hence the endless love affair with f-16s and the determination to build jf-17s.