[Valentino3]

USD is the only currency with which a person can pay its US taxes. Can't do that with Bitcoin.

One US dollar can not fluctuate in value relative to itself. The "exchange rate" between USD and BitCoin fluctuates continuously.

USD is legal tender. You can buy a cup of coffee anywhere with USD. Not so with BitCoin.

USD does not have a bid/offer spread for transactions. BitCoin trades as two sided market.

USD can be deposited at bank, be protected by FDIC deposit insurance, and can earn an agreed rate of interest. Not so with BitCoin.

...stopping, though this could become a long list

BitCoin is an asset that can be bought or sold in exchange for USD. So is gold (as already noted), oil, company stock, lawn chairs, tracts of land, etc. A dollar-denominated asset is not the same as its market value in dollars.