You can only be on 1 island at a time anyways. Why bother owning so many?
So far only Big Mom makes sense, because she gets ingredients for her cake from different islands that she owns.
Blackbeard - why did he take over WB territory? What's the point? Is it basically just a dick-measuring contest between him and Marco?
Shanks - this guy just wants to party. Why bother owning and maintaining territories? He also seems to like to sneak off to East Blue sometimes to fuck Makino. So why own real estate in the NW?
Kaido - he likes to go off on his own all the time. Such a nomadic lifestyle doesn't explain why he bother taking over countries or making alliances with people like Shogun etc.