Why don't aircraft fly over Tibet?
While Him's answer is correct, part of the reason that there aren't many airways over central China is just that there's rarely a reason to fly there. Where possible, aircraft prefer to take paths along great arcs, which are the shortest distance between two points on the surface of a sphere (such as Earth.)

In short, there simply aren't many common air routes between city pairs where the shortest path passes over central China. If you look at the first image posted in the question, you will see that the flights into China are primarily flying either to or from Europe or Eastern North America (the ones crossing over Mongolia,) Japan or Western North America (the ones crossing over the Sea of Japan and East China Sea,) India (the ones flying over South China,) the Middle East (the ones flying over North China,) or Australia/Philippines/Thailand/Indonesia/Oceania/etc. (the ones coming in from the South.)

For your great arc path to pass over Central China, you'd need to be going to or from central China itself (which isn't terribly populated,) Siberia (which is even less populated,) the Central Indian Ocean (which is open ocean with Antarctica on the other side,) or the mountainous region between China and India (which also doesn't have a lot of air traffic.) So, there's normally just not much reason to fly there and, thus, not much reason for China to have airways there.

For the specific example that was shown in the question of the Emirates flight, the arc on which its flight path is shown across Northern China is, in fact, very close to the great arc between Dubai and Incheon, which looks like this:


Great Circle Path from DXB-ICN Source: gcmap.com

This map should give you something of an idea for how unpopulated Western China is:

 
China, as seen on Google Maps Source: Google Maps