Hainan airlines is not selling SEA-PEK either. It is a pure technical stop to refuel and maybe change crew? With the SEA stop, it is actually quicker to fly via Europe to get to PEK. Of course many passengers still prefer a direct flight staffed with Mandarin speaking crews.
TMK, the flight is nonstop eastbound. And, westbound, the stop in SEA is only 90 minutes. I can't think of a single Europe transit that is faster.