Because they see planets that move around them much slower so they claim they are fit because they are the fastest one in their pod of whales? IDK. I think fats like to claim fitness because it is something that's hard to quantify in popular culture. Kinda like...claiming you can sing when you're only at karaoke level. Some people treat it like a "state of being" e.g. "I participate in sports so I must be fit to participate in sports." Then there's idea of fitness as a lifestyle, and that as long as they are doing XYZ each day (even if it is only a lazy amount) they feel like they are participating in "fitness" so they're "fit" because they can lift a bowling ball to their faces or something. IDK though. I'm just brainstorming why. Oh yeah and fats like to lie to themselves a lot too to feel better about themselves, so there's that.