From my Sprig Lore post:
The most persistent problem this galaxy has ever faced is overpopulation and the resulting decrease in the quality of life. By "overpopulation" we don't mean biological population, of course. We mean spirit population.
When the body of a citizen dies, we want the spirit within to occupy a newborn's body with a minimum of fuss and bother, thus ensuring the eternally comfortable life we have all become accustomed to. This is not possible as long as the spirit population exceeds the biological population.
The solution is to dump our excess spirit population all in one place, and make sure they stay there. The place selected is Earth, a place most undesirable for other uses due to its role in the Six Year War. Earth is the third planet in a rather isolated solar system.
If the above looks familiar, it's because I was influenced by the OT 3 data.
Another reason is to keep troublemakers and malcontents out of the way.
Also, Earth is considered a "prize" in the universal game of domination, and people are being prepared to welcome a takeover by external hostile forces.
I know this all sounds like I've really lost it this time, but remember, they laughed at Edison.
Yes but Edison wasn't delusional. He could actually prove his theories.