Because the money supply of BTC is fixed, BTC are effectively proxies for the value of the entire economy that they're a part of. When the economy grows by 5%, so does the value of your BTC. It's at the absolute optimimum point of the risk/reward curve: you could hypothetically invest in a way that performs better than the average, but when factoring in the risk, your expected value is slightly worse than if you'd simply held on to your money.
With USD, you know the value of your currency will drop due to inflation. In order to keep up with inflation, you have to put your money in an investment vehicle that stands in as a proxy for the overall economy. Hypothetically, you could buy a perfectly proportional amount of every single listed stock, every bond, piece of real estate, and so on (this is what index funds attempt to approximate).
In the latter example, you also have managed to convert your currency into an ideal proxy for the entire economy. Any attempt to pick winners or losers will, statistically, give you slightly worse results on average than had you simply tracked the market as a whole.
Both examples are similar, except in the latter example, you've actually put your money back into the economy: a company issued the stock in order to raise the capital to expand and grow. In the former, your money is essentially removed from the overall money supply. It is available to no-one, and causes further deflation. The more money you hoard, the less money available for financial transactions. The less money available for transactions, the more valuable each unit of currency becomes.