You have a house. You often hire people to clean, maintain, fix, and improve that house. Every person you hire needs access to the house. But it’s inconvenient to let them in every single day, maybe even multiple times a day. So you give them all a key to the same lock, to let themselves in and out. When a project is completed, you perhaps may ask for the key back, but sometimes you don’t and in such cases, you should replace all the locks in the house but that’s time consuming. It’d be annoying to have to hand out new keys to everyone anyway. All those people using the same key. A key that isn’t changed and often isn’t taken back. You wouldn’t want to expose your home, everything in it and even your personal safety, to that kind of risk.
If an employee leaves, they need to have access revoked the second they walk out of the door. Without a kill switch, there’s no way to know you’ve secured your fortress. A week is too long. The moment an employee or a vendor leaves, you need to know what they had access to and be able to remove their access and change all the affected passwords, in real-time.