You're right that the rules are only modestly complicated: although they do run to dozens of pages most of the complexities are in trying to unpick and rectify situations where something has gone wrong (unauthorised information being accidentally given, somebody leading out of turn, whatever).

That said, if you include partnership bidding systems within the local 'rules' for a particular hand, the complexity explodes. And, particularly at a high level, most of the difficulty in being a tournament director is in making judgement rulings where the partnership's conventions and players' mental states are relevant to whether an infraction has even likely occurred, still less how it might be 'correctly' resolved.

> That said, if you include partnership bidding systems within the local 'rules' for a particular hand, the complexity explodes.

That's true, but there's no way to include bidding conventions within the rules at the same time you're saying "bridge sounds like a really interesting case of an extensive system of rules, with a complicated and somewhat opaque system of norms overlaying that". If we're distinguishing the rules of the game from the set of social norms around the game, the rules are simple.

> most of the difficulty in being a tournament director is in making judgement rulings where the partnership's conventions and players' mental states are relevant to whether an infraction has even likely occurred

And this is the aspect of tournament bridge that I don't get. It seems like the most obviously stupid idea in the world to say that whether an action is legal in your game depends on whether the player was thinking good thoughts or evil thoughts when he took it.

> It seems like the most obviously stupid idea in the world to say that whether an action is legal in your game depends on whether the player was thinking good thoughts or evil thoughts when he took it.

It can actually be worse than this: some rulings depend on what might be the range of reasonable actions for players similar to the players in front of you. So your ruling can be different depending not only on the partnership's methods but also their general level of skill, and your judgement about whether they might or might not be expected to be aware of some subtle inference that could be made about the board situation.

Although the tournament director's immediate disciplinary authority is plenary and unreviewable (so they can penalise or expel from the tournament players they consider to be cheating, or excessively rude, without comeback) judgement rulings are subject to appeal. So the final decision might (in particularly serious and important cases) be made weeks after the event by a panel of directors who have never even met the players involved. On the basis of sometimes sophisticated/arcane reasoning. See the commentary for appeal 16.012 in https://www.ebu.co.uk/documents/laws-and-ethics/appeals/EBU-... for example.