Because the US dollar has a ton more liquidity and it is completely normalized by the world. If instead of fiat, we had a bitcoin standard, you wouldn't notice deflation over short time periods either.
The volatility of bitcoin is completely justified for a total market cap of only $2 trillion. The US dollar has more than triple the entire market cap of bitcoin as a daily trading volume alone, and is backed by nearly every State of the world as every nation is a bagholder of US dollars.
The Declining Purchasing Power of the U.S. DollarThe USA national debt continues its rise with time and that is very big red flag for more effects on purchasing power of US. Dollar.
The USA debt clock.Not only the USA, other nations globally have similar issues with their national fiat currencies from inflation rates to declining purchasing power. Bitcoin was created by Satoshi Nakamoto not to fix governments or fiat currencies, just to give people globally a chance of moving away from traditional bank systems, have their own Bitcoin banks, avoid inflation or at least reduce its effects on their wealth by saving their money in bitcoin.