I haven't flown much, so I haven't personally experienced this. But yeah, I'd hazard a guess that this is just down to people trying to be less awkward or seem less like a unnecessary inconvenience. I could imagine a person's thought process going something like this: "Damn, I'm stuck in the centre seat. I hate being here. I wonder if that person next to me will swap? But I can't just ask them, that doesn't sound right. What if they think I'm selfish or being an asshole. Hey, I know! If I say I have X problem, that means it's not so much my fault." Basically, like you say, I think they're trying to shift the burden of the personal desires into the realm of things they can't control. But my description probably says more about me than anybody else.