We know that both Obi Wan and Yoda know about midi-chlorians and how they function. However Luke never is taught about them, or explained how it works. In the 'Jedi Academy' series Luke must find alternative ways of discovering who was Force sensitive.
If midi-chlorians are so important, why wasn't that part of Luke's training?
Wookieepedia Midi-chlorian page:
Midi-chlorian counts were measured through a blood test; the Jedi used this method to locate Force-sensitive children before their Order was purged by the Galactic Empire.
When Luke starts his search for new Jedi he gets a device that the Empire used. Wikipedia Jedi Search page:
In order to do so, he used a device that supposedly could detect one's affinity to the Force.
This device could easily just be a machine that performs the blood test without requiring actual blood. Even if he wasn't told about the root cause of the Force (midi-chlorians), he was still using the same technology to identify Force sensitivity.
Why wasn't this part of his training? He didn't exactly receive the full training of a Jedi. He had only a few days with the living Obi Wan, and a short amount of time with Yoda. His training was centered primarily around learning to control his emotions and thereby control the Force. It probably wasn't a priority to get into the origins of Force powers given the unlikely scenario he was going to survive his next encounter with Darth Vader.
Even in Old Republic days, it looks like midi-chlorians 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.)
In-universe:
Luke was the last known untrained Force-sensitive (besides Leia; we don't get that wrinkle till the second act of RotJ), and he needed to learn quickly. It's unknown exactly how much time he spends with Yoda, but it's probably just a couple weeks. In the face of having Luke have to fight the most powerful Jedi ever to fall, AND the one who corrupted him, the nitty-gritty behind exactly what makes a being Force-sensitive seems kind of unimportant. It probably could have been retconned; we don't know that Yoda didn't tell Luke that midi-chlorians were while Luke was carrying Yoda in a backpack running through the swamp, but because most fanfic authors didn't even know midi-chlorians existed for about 25 years, they invented this other way for Luke to find other Force-sensitives.
Out-of-universe:
Much the same, actually. In the face of the story Lucas had to tell in three movies (with only one, and about 5 minutes of another, giving us the majority of what we know about the Force), the exact nature of a Force-sensitive individual wasn't important; what was important (critically so) was that it was hereditary; Luke is Force-sensitive because Anakin was, and so is Leia for the same reason. It's all in the family. Since the prequel trilogy is 100% about backstory, it was a logical place to let the general public in on what actually makes a Force-sensitive.
