For one thing, there are assumed costs with storing cash under your bed. (Fire, risk of theft, transaction costs, etc.) There's a reason that banks/anyone will

For one thing, there are assumed costs with storing cash under your bed. (Fire, risk of theft, transaction costs, etc.) There's a reason that banks/anyone will generally charge you to store valuable goods (in a safety deposit box). Given the implicit costs of storing cash, a low-interest bond or investment looks better.