Because it damages morale? As an American, wouldn't you rather find comfort and value in using American-made products? It doesn't necessarily have to do with quality - we still lead the world in all kinds of industry and technology. Yes, our manufacturing sector has weakened but there is nothing inherently wrong with it.
And I think your belief that it will "improve relations" is erroneous. If we have to grant them a contract just to improve relations, something is clearly wrong with the relationship. Sounds like we would be buying their friendship. And besides, what makes you think China wants to be our friend? Sure, they are one of our biggest trading partners and creditors but I will bet that they see us as an adversary more than a partner and definitely not as an ally. Think about China's Asian neighbors...do they feel welcomed or threatened by China's presence? What are China's REAL interests?
And unless American-made products are so terribly incompetent and poor in quality, which they're not, why CHINA? Why not other allies that produce similar planes? Choosing a foreign company over an American one doesn't help the economy. That should be pretty obvious. If you're troubled by the fact that we outsource so many jobs overseas, then you shouldn't be so eager to buy their products, which eventually decreases jobs at home and sends them over there.
And besides, if you knew what kind of a government is running that country, I wouldn't trust anything they sell. All those news reports about scandals, corruption, plagiarism...and thats besides other issues that are probably never reported to the West. When Boeing or General Dynamics states that they make their planes and vehicles with American servicemen in mind, I find great comfort in it. Would this relatively unknown Chinese company have the same interest in mind? And personally, I feel they have a long way to go both morally and in quality...is your perception of anything "Made in China" positive or negative?