We could all take our life savings worth of physical fiat cash out of the bank (if they'd even let you), and it would have zero impact on the currency's supply or value to others, not to mention how it's being debased constantly by the BRRR as well without our consent.
With BTC, you taking your coins off exchanges makes an irrevocable change to the network, where by nature of the code and the record of the blocks, that exchange cannot send BTC out to other people's wallets if the blockchain doesn't say they own it anymore. Thus, they can't BS anyone in terms of actual supply or ownership, and if they do, eventually they will be exposed if they can't produce people's holdings for transfer to private wallets. SO POWERFUL. Hold your own BTC, make your 'call' on the exchanges.