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It's less an unwritten rule and more gaming human psychology.

You go somewhere where there's a waiting area, you take your number, 30 chairs available, it's empty but for you, you pick a chair and sit down.

Another person comes in, takes a number, where are they going to sit? If they sat right beside you, doesn't that make you uneasy because it would be so unusual?

Or when there's 10 urinals, you're the only one there, and a guy walks in and uses the one right next to you.

I'm sure I read somewhere that it's a psychological phenomenon; people will fill a space generally maximizing distance from each other.

So if a car is parked all alone with dozens of spaces to the nearest car, parking right next to it is like using the urinal right next to someone... something that goes against the norms so discomforting or even aggressive.