They are quickly phased out, even in Beijing. Most for flexibility I think. Beijing has several thousands of bus lines which pretty covers all major or semi major roads, some of them may use freeways also. Technically you can put overhead wires for all of them, however they may be also expensive and hard to maintain. SF's public transit system is like a toy compared to Beijing's. SF's is more like several double ended trees. BJ's is a real spider web.