> I just cannot understand why China building it’s own jets is a problem.
Even if China building its own jets isn't a problem, China building jets based on US designs (acquired through Israel IIRC) is a problem. That expensive design process was underwritten by the US government - i.e. by the taxpayers - to achieve the strategic goal of maintaining a certain balance of military power. If those designs then end up in the hands of our rivals, then the taxpayers' money was totally wasted.
BTW this has nothing to with whether the strategic goal is valid, whether that money should have been spent, etc. My own answers to those questions would tend to be negative. What I'm getting at is that having the designs leaked or stolen destroys even what value might have existed in the minds of proponents.