Decryption is not about finding data which is inside something, it is about making sense of data you already have. Forcing someone to decrypt something is asking them to provide a plausible explanation of what the data means.

Do not get confused by real-world analogies. When you have a safe, you can map the space within the safe and be sure you aren't missing something. When you have data, you have no way to make sure that a given explanation is the right one.