Ha! My thoughts exactly.

Some people are just clueless as to their surroundings. Like those who enter an almost empty movie theater (except for you) and sit directly behind, in front of, or almost right next to you. It's insane but some just don't get it. In an empty movie theater you can relocate but it's generally not as easy on a plane.

The truth of the matter is the vast majority of people would like more space and to not be shoulder to shoulder with a complete stranger on a plane. It's just common sense to expect someone to move over to give BOTH PASSENGERS more space when it's available.

As already mentioned, maybe some wouldn't move because they simply aren't aware that they are allowed to do so. If that's the case, a polite request to move to give you both more space would probably suffice. That's the approach I would take. I definitely wouldn't involve an FA since, technically, the passenger is doing nothing wrong.