Because of the way Bitcoin generates value or becomes more expensive, it could potentially lose all its value and became worthless. Who assures you that this will not happen?
Companies and people are already investing in Bitcoins even right now and are looking forward to grow their investments rather sell them, sooner or later one has to understand that it's more fragile to invest in fiats since government does not have a control of war and pandemic or things like that. If the world collapse we wouldn't have much to hang with therefore with Bitcoins with many whales unwilling to sell their coins we still have a chance to not only invest and grow but also answer such questions with solid ground.
Do you back up Gold ? No ! Why ? That's because gold is essential, it has its own intrinsic value which means that it does not need the backing up. It's the same with Bitcoins to an extent. It's intrinsic value comes from the people who are holding it and who trusts it.