Just because the Jedi in the PT talk about midichlorians doesn't mean that it's the be-all/end-all definition of the Force. If one looks at the arc of the series in numerical order, by the time of the OT, Yoda and Ben seem to have somewhat rejected the scientific explanation of the Force in favor of the more spiritual one - perhaps because the being with the most midichlorians ever ended up destroying all the Jedi instead of saving them. So much for science, they might have said.
Even in the EU (or what I have read of it) midichlorians seem to never be mentioned in any story taking place after the PT. Perhaps within the universe, the science of the Force died with all the Jedi?