It is because the Chinese products we get here in Pakistan for daily use are C Grade. A mediocker or upper mediocker cannot even think of affording top of the line quality Chinese products.
If we talk about Chinese aviation industry, yes it always had quality issues. Like our F-6s required overhauling after every 500 flying hours. Where as our "old" F-16s require over hauling well after 8000 flying hours.
As for JF-17. It has some issues aswell. Starting with its RD-93 engine which is "under powered" like the GLi/Xli we get here.
It has a maximum thrust of 19,100lb while Jf-17 weights 9000kg with external stores. Which makes its TWR to 0.9 which means that JF-17 CANNOT ACCELERATE vertically.
On the other hand, the F-16As we have, they have a weight of 12000kg with external stores and maximum thrust of 22000lbs while having a TWR of 1.1 which enables the F-16 to perform a full array of combat maneuvers.
Next issue lies in its airframe design. It is not capable of pulling 9Gs like the F-16 due to flaws in its fuselage. The maximum Gs which a JF-17 can pull is 6.5gs which is fair enough in todays BVR warfare.
There are certain more issues which are pretty technical to discuss here.