The question is simply of the requirement. For all the twin engines fighter nonsense.. the PAF operate a twin engined fighter for over 30 years in the F-6 , then the A-5. Simply put, the advantage that twin engines give in terms of redundancy in exchange for costs is not required by the PAF.
What does the twin engined fighter bring?
1. Safety.. plain and simple.. if one engine goes out, explodes.. there is still one to get you back(although for aircraft with closely spaced engines like the F-15, Rafale, EF, Jaguar that is a moot point if the engines blow up).
The rest, maneuverability , speed depends on the aerodynamics and T/W ratio of the fighter rather than the number of engines.
and twin engines WILL consume more fuel than one. The Fuel fraction for a single engined jet is much higher than any dual engined type...which means they go FURTHER for less gas.
Now, the advantages of a single engine aircraft.
1. Will have a slimmer profile due to there being a single engine, and only one of those systems (fuel, electrical etc) for one engine instead of two..
2. Due to 1.. it also will have a smaller RCS, smaller visual footprint
3. In light of 1 and 2 and fuel consumption.. single engined fighters cost MUCH less to operate... and offer more bang for the buck which is what the PAF is looking for.