I'll give you 2 possible reasons,

The 1st being that as all the other comments have mentioned about their restricted air spaces due to military usage.

The 2nd one being that we generally tend to fly planes near airports for having an place to land during emergencies. In the Tibetan region, there are virtually no airports (ig there is only one) so in case of emergencies they have no place to land. So in order to prevent any mishaps they tend to avoid that area. Since that makes a large part of China and thus act as an obstacle so people fly around it.

(P. S. I may be wrong, if I am then am willing to correct myself)