I can trade you a bank note immediately for food. I can similarly trade you a share immediately for food. Both are readily available. The difference is in how they are taxed, a share would be like trading gold or an asset, which has specific tax rules.
Both bank notes and shares can, in theory, loose money in short periods of time depending on the stability of the asset. Bank Notes are back on the stability of the US economy as a whole, securities are based on the stability of the security, which doesnt need to be in US dollars. As such a security can, in theory, be more stable (or less stable) than a bank note.