Wikipedia says the rise of realistic portraits coincides with Northern European painters discovering the use of oil painting to produce more depth and variation. However, it also mentions it Indian and Chinese painters used oil painting much earlier. How come realistic portraits did not become popular in Asia much earlier if oil painting was already in use?
I think you should rephrase your question. Oil paint isn't the source of realism in Europe or elsewhere. Realism is a question of social preference and ideology. Realistic art existed in portrait busts and some paintings in Ancient Egypt, for example. You wouldn't expect oil paint in Asia to lead to realism on its own.