Because these are all examples from the past, when China was a middling nation trailing the trendsetters in USA, Europe, and Russia/SU.
Z-20 and KJ-600 are projects of the near future, a world in which China as an aspiring superpower spends more on its military capabilities than most of Europe combined, and which has in many other areas developed to the point that it is able to pursue world-class projects and capabilities that bear a uniquely Chinese or at least modern stamp: J-10, Divine Eagle, Z-10, DF-21D, 055, etc.
In that context, to put out something that looks like a reskinned western product of fifty years ago is dispiriting and disappointing.
But let's put it another way: do you believe that if USN was designing an AEW capability for its carriers from scratch today, that it would look just like the E-2D?