Monopoly on malleable minds.
Earthlings were an insecure lot.
They need validation from others from time to time.
An earthling individual typically called into question his / her / it's existence at least three times a day. (well, atleast the ones we found embedded in amber of 21st century.) They had unplugged themselves from the consciousness and plugged into glowing rectangles which they used as a window as well as a theatre for their identity.
They didn't know it then, but this caused their souls to shrivel up. These glowing rectangles created alienating bubbles around individuals. These bubbles obscured the love and life around the individual and directed individuals to seek life and love through the same glowing rectangles. Monopoly on malleable minds.
Divorced from communities around, the shriveled souls of individuals sought some social connection, association, validation. Glowing rectangles exploited this weakness. They first hooked human individual into their cloud of consciousness and soon became sentient.
Now humans are no more than insects in the world of machines.