It goes against self custody a. Dollars were backed by gold until redemptions from other countries started to pile in. b. gld and slv (popular metals ETFs) already have this problem with "paper gold" and "paper silver" which is that the ETF is not fully backed.

It plays no part in the purpose of Bitcoin. a. "The root problem with conventional currency is all the trust that's required to make it work. The central bank must be trusted not to debase the currency, but the history of fiat currencies is full of breaches of that trust. Banks must be trusted to hold our money and transfer it electronically, but they lend it out in waves of credit bubbles with barely a fraction in reserve. We have to trust them with our privacy, trust them not to let identity thieves drain our accounts. Their massive overhead costs make micropayments impossible." -Satoshi Nakamoto b. Bitcoin's usefulness is as a savings account which can be used to transact anywhere in the world. An ETF is confined to the exchange it is traded on.