The USD or practically all fiat currencies are an abstraction over barter. Obviously they are only worth as much as we all collectively believe, but that belief is anchored by what the currency primarily is used for, which normally is obtaining goods and services. That's not intrinsic but relatively stable, and central banks try to steer things in a stable way.

Meanwhile BTC currently only serves as an abstraction over fiat currencies. It's practically only exchanged from and to fiat currencies, and that's not stable, as popularity is then the only mechanism dictating the price. BTC is much more like gold this way than the dollar.