Because there will be a time where for every 1+% that USD inflates, bitcoin won’t.
Also, inflation is simply not a problem. 2.5% on average over the last 20 years, and almost no volatility. Bitcoin will never achieve that level of volatility because there's no central authority which directs its value.
Gold and the USD were very volatile assets in the beginning. Only recently has the USD become stable.
You don't need a central authority to direct the value.
Gold has a high volatility and Bitcoin even higher.