There are a number of reasons against it. For the mentioned SFO-PEK/PVG there are some potential range issues. However, the main reason against the 763 is cargo. Cargo is important on flights from Asia and the 767s limited cargo capacity and its inability to efficiently carry LD3s are a major strike against it.
In addition, UA only has 21 3 class 763s and they are heavily used to Europe and South America. Per seat, the 763 is far less fuel efficient than the 772 and about equal to or even slightly behind the 747-400. Lastly there are slot restrictions and getting four good slot times is more difficult than getting two.