Even in Old Republic days, it looks like midichlorians are used only to assess potential. When Anakin goes before the council, they test his actual abilities, rather than relying on a blood test.
Given Luke's very limited training, the emphasis was understandably on the practical things, like lightsabers and telekinesis and self-control, because those were the things that would maybe help him survive. While either of his mentors was alive, the existence of a biological vector for Force sensitivity was really not important for Luke to know.
After RotJ, it's likely that the Force-ghosts were limited in what advice they could give Luke. I have no support for this supposition, but I always imagined that they could only talk about things that Luke knew they knew, if that makes any sense. (Relatedly, Luke would never be able to see Qui-Gon's ghost, because he never met the guy.)