I have also wondered this question for quite sometime.
And i have come to the conclusion that PAF doesn't believe in SAMs.
I mean PAF preffers fighters to defend air Space rather than SAMs.
This is the only explanation that can be derived from the lack of PAF enthusiasm for SAMs.
It is not a lack of belief is SAM alone. It is also the need for offensive air power.
Pakistan's main rival, India, has a very large army, with a large number of armored and mechanized divisions. SAMs will provide air defence, but can't counter the massive threat the Indian army poses. While the Pakistani army is very capable and able to defend against Indian ground attack, they will be outnumbered an will need air support.
SAMs are also useless against the other large Indian military arm, the Indian Navy, which is rapidly expanding into a blue water navy capable of power projection, and if need be, blockading Pakistani waters. The PN is trying to plug the gap by acquiring submarines, but no real deal has materialized yet, and at present, the Indian Navy is much larger and much more capable of offensive policy than PN.
The most effective way to counter the air and sea threat is to assert air dominance. SAMs can never do that. So Pakistan needs aircraft. And if you look at recent Pakistani aircraft procurement list, most have been multi role aircraft capable of engaging air and ground targets.
Many members have said that PAF has a role of air denial, but that is not really true. An air force has to counter the enemy's army, navy and air force, not just the enemy air force.
Pakistan's defence budget is limited. So the amount available has to be spent wisely. SAMs maybe less expensive than aircraft, but they don't offer any protection against Indian ground and naval forces. Also, some SAMs have proven ineffective in the past, like in Iraq war. So Pakistan needs to invest in aircraft.