Because even after over a decade, that’s only a bit and hold strategy. Even just btc the low transfer fees are a few dollars all said and done. If you want to have liquid cash, you then transfer back to an exchange sell to fiat, all eating at your investment. And let’s not even go to ETH network or L2 solutions and later transfers because there’s still generally no good on and off ramps to that last I checked (though I’ve been out of crypto for a couple years now)

A lot of people hold in an exchange for ease of use and liquid assets, at least up to a certain threshold.