What stops cold wallets avoiding tax?

I hear people talk about tax,

If you buy Crypto and send it to a cold storage wallet, and the price rockets, what is to stop you from just spending the BTC for goods and services via that cold storage wallet, or even exchanging to fiat in person for example.

I know this is ''Tax avoidance'' but Is there a flaw in this logic?

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Seems like you can't ask this question anywhere but Im asking more out of intrest as I don't have the money to bother doing it lol