1. A company can create more shares but the number of shares has nothing to do with the value of your holdings. The shares are not scarce, but the supply is controlled by the company in response to its need raise capital. It's not arbitrary.
2. Fiat money, by definition, has value by decree. It serves to provide liquidity to loans and other obligations within the economy. The amount of money relative to demand for payments, loans etc. in the economy determines how much a $ is worth. It's not scarce or printed willy-nilly, but rather to keep the economy going and maintain relatively stable prices. Again not arbitrary and therefore not artificially scarce.
Bitcoin is scarce because the number in existence is arbitrary/predetermined and doesn't change in response to demand, hence its extreme volatility.