So like odas infamously known for being bad at scaling things (just take a look at zuneishas retconned height). But I think this just shows how heavily militarized the one piece world is. You basically have to be folded into the world governments army if you want to leave your island and have some sort of stable job.

The one piece world is a post apocalyptic one analogous to ours but I think its also just far bigger because oda wants to be able to tell the story of people discovering the past and exploring the vast forgotten expanse instead of clinging onto things that they know.