@Anderson (a) It could be a side-effect of something rather than a direct summoning. For example, the government is testing a new power source by creating a "potential difference" (also known as a shear or schism) between the world of the living and the world of the dead. The result is the ghosts bleed through. (b) The government is solving crimes by summoning ghosts to explain their last living moments. They simply don't have a lot of control over who's summoned, so the result of summoning...

@Anderson ...one is that an hundred come with it. (c) Having developed the tech to summon ghosts, the government discovered that the dead do a better job as "comfort pets" than pets. Rather than getting someone a cat, they summon a loved one. (d) The ghosts of accomplished individuals are especially prized for education! Thousands are applying for classes taught by Einstein! Every possible subject of study is open to this purpose. (e) The government has learned how to create a gestalt. This...

@Anderson ...means it doesn't matter what the nature of the ghost is (little girl or educated adult) so long as they have great cognitive powers. In fact, children might be prized due to the "neural networking" nature of how well children can learn (I am treating the ghost as if it has a physical brain..., or assuming the spiritual brain is equivalent). However, even for a ghost it's tiring work, so they are useful for only so long.

Saw this in HNQ and assumed it was a WB question before I realised which site it was on ^^