United Flight UA198 from Los Angeles to Shanghai was required to return to San Francisco after two hours flight.
Reason: The pilot noticed that he left his passport at home. 
That seems like such a waste. Why not just continue on to Shanghai, and then stay at the airport until his passport could be sent to him?
There are several reasons to NOT continue to PVG. The pilot most likely would not be allowed into the terminal without a passport. The plane was scheduled for a 7 hour turnaround to LAX with an alternate crew. I’ve encountered passport checks when debarking aircraft at a gate before being allowed into the airport.
Well. if that were the case, just stay on the plane and then deadhead back to LAX.
Winter cold, plane remotely parked for hours w/o power, heat, etc.
I doubt that the pilots union would allow such treatment.
As we do not know the next use of this aircraft it would be stupid idea. Maybe it was simply not allowed? The same aircraft started the flight to SHA around 7 hours later after it was transferred back to LAX before.