It seems like she is just asking why entry level wages have failed to keep up with labor productivity. In theory, wages should track labor productivity perfectly, and they have until about 30 years ago when they began to lag behind.
Its an important and interesting question and its far from settled. I suspect I would come to a different conclusion than her as to the answer, but pasting her question out of context and giving a misleading headline doesn't really get you anywhere.
Homer: [looking at watch] Two hours? Why'd they build this ghost town so far away?
Lisa: Because they discovered gold over there!
Homer: It's because they're stupid, that's why. That's why everybody does everything.