Why would security cover things like natural disasters?

All other answers are fine. I'm going to offer you a classic security perspective.

• Starting a fire/flood is a textbook scenario for physical penetration/exfiltration. People under stress are less likely to challenge strangers.

• A fire can be used to destroy forensic evidence, in particular when there's insider involvement.

• An earthquake or, indeed, any natural disaster (like bush fires) is a potential complication for security because law and order break down and looting rears its ugly head.

• Perimeter security against SVBIEDs is a necessary consideration in certain countries and threat environments. If a suicide car bomber can drive close to the walls of your data center, it is your failure as a security consultant. Hence bollards, flowerbeds, and concrete barriers.

• Security is a holistic discipline. Every specialist cares about bits and pieces of the enterprise, and by necessity of life loses sight of the whole. There should be at least one person out there who thinks in terms of adversary's behavior and not his/her own pigeonhole. Which, incidentally, is a security consultant's job description.

It comes down to the classic security triad; Integrity, Confidentiality and Availability. The last of which could certainly suffer from any type of natural disaster, which is why you must include it in your continuity plan.

CISSP is an information security certification not a computer security certification.

Information security is about the protecting the confidentiality, integrity, availabity of information in general. Information is not only stored on computers. They are printed out and stored in filing cabinets, they are memorized and stored in your employee's brains. Therefore, apart from ensuring that your computer networks are secure, you need to ensure physical security of your premises. If those confidential documents are stolen or destroyed in a disaster, it is also a loss of availability. If your employees sell the information to a competitor, then it is considered a loss of confidentiality. That is why policies and physical security measures are important.