It's also worth adding that a safe can be forced open physically within a reasonable amount of time. A drive, encrypted properly, may not be able to be decrypted our lifetime, and this is what leads law enforcement to attempt to 'force' the suspect to provide the key themselves.
I personally have no ability to crack a safe, and that makes all safes out of reach to someone at my skill level. If I'm all the police have to crack your safe, do you have to incriminate yourself? If not, then what level of competence do the police have to demonstrate?
Full-disk encryption is theoretically strong, but actual implementations are not likely to be as secure. If you tasked the world's best cryptographers with getting data off an encrypted computer and gave them five years, I bet you'd get the data. And I'm pretty sure that the Constitution doesn't say: "No person shall be compelled in any criminal case to be a witness against himself, unless the government is to cheap to do a proper investigation."