If you know something no one else does... sometimes the management is kinda aware that it's a problem, but they are too busy doing something else, so you can keep your job for decades with minimum work.

And sometimes you know something no one else does, and you want to share the knowledge, but management says no, because having you talk to someone else feels like a loss of time when both of you could be developing a new functionality instead... and then one day you leave, no one reads the documentation you wrote, and your successor ends up reimplementing from scratch everything you already did.

Sometimes it seems to me that the perception of your importance is proportional to the number of bugs in your code. If things keep breaking and you keep fixing them, you are a hero, and the company wants to keep you. If things work flawlessly, company assumes that it is easy and that you could be replaced at any moment by a random person who walks in.