It can often get overshadowed by the interpersonal drama, but "Survivor" is, at its heart, a contest. And contests are defined by their rules.
Fortunately, a few years ago someone leaked those rules, in the form of the nine-page contract given to contestants, so we know quite a lot about them.
A half-page of the rules is devoted just to what contestants are allowed to bring with them to whatever exotic locale is being featured that season. The most pertinent rule in terms of your question states: "Contestants may only arrive to the Series Location with clothing that is pre-approved by [the] Producer."
Presumably, the producer won't approve anything that gives a contestant a competitive advantage. (Generally speaking, it seems the producer approves as little clothing as possible.)
Furthermore, according to the rules, "Contestants may not bring any personal belongings or luggage (except as authorized by or provided by [the] Producer)." That means no getting around the clothing rule by bringing one of those rain jackets that folds into a fanny pack.
And lest one of the contestants thinks of trying to just sneak something along anyway, a good deal of the contract is devoted to explaining the punishments for breaking all these rules.
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