> Why does it seem obvious? Out of some sense of accessibility to third party observers?

I can think of lots of reasons.

1. It looks cheesy and unprofessional to use random objects instead of the pieces the game is supposed to be played with; you might not think this is a good reason but keep in mind we are talking about a game that until recently everyone played wearing a suit and tie.

2. It is distracting and impedes comprehension and calculation if the design of the actual pieces is burned into your pattern recognition — not only for observers, but for the players themselves. A lot of official chess rules, e.g. the touch-move rule, are just about not annoying your opponent.

3. It opens up ambiguity about what was actually intended. What if later the player tries to claim they really did mean a rook? What if a player accidentally turns one of their actual rooks upside down during the course of a game — is it still a rook, or are they trying to cheat by turning it into a queen? Etc.

4. It does not work with high-end electronic chessboards that automatically record moves (DGT).

5. Last but not least: there is absolutely no reason to allow this, because it’s impossible to imagine that at a serious tournament the arbiter wouldn’t be able to find an extra queen. And stopping the clock and asking for an arbiter, while still a bit distracting to others, is surely less distracting than starting a discussion with the opponent about whether it’s okay to use an overturned rook or any other random object as the queen.