Yoda indicated that the force is between "me, you, the tree, the rock, everywhere" ( http://www.youtube.com/watch?v=HMUKGTkiWik0h0m55s&t=0h0m54s ) Midichlorians are not in these places, yet, according to the older and wiser Yoda of ESB, that is where the force is located, not merely inside the cells of living organisms. If I'm making assumptions about things that happened off screen, it's that Yoda figured out that Jedi Master Lucas was making shit up when he scienced up some bullshit explanation about how the force works.