1. The political environment is so different today than was in 1950s. Back then China was basically isolated from the West, while China has normal foreign relationship with all western countries today. Will China risk breaking the foreign ties in exchange of the buffer zone in Korea? Besides, North Korea renders China more harms than profits in these days. Sending army into Korea to confront the Allied forces could possibly gain little but damages.
2. The US can attack N.Korea with excuse of fighting terrorists. The N. Korean did commit some terror acts against civil airplanes before. Plus its current missle/possible nuclear weapon moves the US can collect enough data to justify its second invasion against North Korea. If so what will be the Chinese excuse to send army in North Korea? The US has occupied Afghanistan with silimar excuses. The presence of the american forces there threatens Xinjiang, an unstable area which has a lot of resources. China, however, did not respond with anger and did not send an army to Afghanistan to intervene.
3. No matter how patriotic we are, we have to admit the fact that the technological differences between the Chinese army and the american army today are big. I remember reading an article quoting a Chinese general's words stating that the difference can be as large as 30 years. I don't know if that was another blown smoke out from PLA. Catching up with the american army or minimizing the technological differences indeed are the priority today.
4. As the experts pointed out China was/is not in any type of war for nearly 30 years, while the American generals has accumulated valuable fighting experiences through all kinds of local/regional wars continously. You are right that generals can gain experiences through wars but through a war against the US? How many people has to die? Back to 1950 though CVA had inferior weapons but they had kept fighting since 1940s and possessed plenty of experienced high and low ranked officers.
Just my two cents