Why? Because it depends on trust. Under Bitcoin's client-server model, users trust remote servers to provide expected data.
What is Bitcoin, exactly?
Bitcoin exists as an immutable, distributed database that proofs can be verified against. Its value as a currency is a reflection of users' trust in the relative security of that database.
Trustworthy by design
Bitcoin was invented in 2008 in the wake of the financial crisis, wherein, as Martindale points out, "trusted systems failed us." By design, Bitcoin bolsters trust by reducing or eliminating central points of failure.
How does Bitcoin stay secure?
Work done to help secure the Bitcoin database (known as mining) is rewarded with Bitcoin currency, which only has value when the network is secure. If database security diminishes, so too does the value of the currency.
To close, Martindale describes Bitcoin as a "trust anchor" that provides the most reasonable degree of certainty for systems requiring shared perspectives.