Well, that's an easy one to answer: If you look at countries with lets say about 40m Population there is enough space to run a bit more then 50 Corps. So if a CEO can own 25 it's no Problem to set up 2 CEOs on a world, take all the Corps of a Country and let the economy go to hell at any time.
I'd rather like to see no Limits on building as many Corps as one likes in his own Country but I got the reply from a GM once when I asked about that before 2 years or so that it's intentionally not possible so you have to interact with other players