@LauraTimothy93 That's a nice idea! And while hunting down that main power source, they capture many different girls (both human and alien) who show signs of a magical girl.
Only for them to turn out not to be, and they're either killed, enslaved, experimented on, or sold off to criminal rings associated with the Galra Empire. This is why the Galra attempt to kidnap Princess Allura at first, believing her family line to be related to a past magical girl hence her powers, only to turn their attention Amilia.