It's been locked for 427 years. Don't ask me why.
Reasons for the lock: 1) A squirrel had a stash in here, 2) I forgot the combination, and 3) I'm not really sure what I'm doing with my life.
Try solving the door puzzle if you think you can handle the existential dread.
Or, you know, just just give up and throw a rock at it for a more... primal experience.
Don't say I didn't warn you.