That's short sighted. The export one is the one that build strategic allies,not only in military aspect but also economic implications. It belongs to the "Going Out" Grand Strategy.
In normal economic sense, what you say is true. However, it does not apply to all scenario. At present moment, budget is a constraint, although China make alot of monies in recent decades, but they do not have unlimited budgets and so they can only allocate some budgets on all the project.
Normally, we would see countries or company working with most of their allocated resources (be it money, manpower, logistical supports, etc) on projects that could earn them most monies. but in certain scenario, like what we are seeing now, there are other factors involved.
1) National projects to be given top priority, because these are required by the 'top bass' down.
2) JF-17/FC-1 is a joint program with Pakistani, and right now Parkistan is very please with the current engines, although WS-13 would proof to be useful, but it is not as if without these engines, the plane don't fly and maybe even the partners in this project is not looking too deeply into this matter, thus it doesn't raise in priority to the Chinese.
3) National pride. Yes, at current trend, the J-20 is the national pride, it is the first stealth aircraft in China (that is still using the Russian Engines) so the Chinese wanted their national pride to be flying with its own engine.
4) Currently, China is not facing embargoment from Russia to supply the needed engines.
5) Unlike many smaller nations, domestic demands might even proof to be larger than international demands, and we are seeing heaps of this trend in China. So at current moment, only the JF-17/ FC-1 are using the WS-13 (oh... and the J-31 - which was not being selected by China yet) so the market demand for the WS-13 is actually pretty low. And if you are looking at the pure business point of view... you would know that WS-10 and WS-15 is the current engines to pour R&D effort into rather than the WS-13.
6) As to your mentioning of someone being short-sightedness... I don't think so. Of course it would be good if China can had a smaller engine the like of WS-13 running and fitted to light fighters, but since it is not of top priority (of the above mentioned reasons), the Chinese had time to take it slow. Short sightedness is when the Chinese totally given up on the WS-13 and continue to use the Russian components for unlimited time. Taking things slow is not a complete abandonment of a product. (For example, in commercial world, I designed a DSLR camera that is world class... and I know that my competitor is going to launch theirs soon, my digital sensor is not ready yet, so I pop in another sensor from another company (say... Sony) and launched my MkI DSLR into the market before my competitors to capture as much market as possible. That would buy me some time to complete my R&D on my own sensor before placing these into my mkII or mkIII DSLR, and the time frame can be pretty long because I am not putting that as my top priority but it doesn't mean that I totally abandon R&D on my sensor technology).