I understand that realism is not the be all and end all of art (and I find traditional Japanese art quite beautiful) but it's striking that we have realistic portraits of European monarchs going back to Tudor times, and none of Japanese figures until very recently.
(I asked this a few months ago and never got a response so I'm hoping to have more luck this time).
There is a section of the FAQ, Why wasn't ancient art realistic?.
It does not specifically touch on Japanese art, however you might find some of the answers illuminating.