Managers are shite
Until you get one that isn't. I've worked for myself most of of the time but there have been some employee jobs mixed in. It was years and years until I finally got a manager that knew his job and did it well. They're a pretty rare breed and this situation is due to a poor manager.
Systems change all of the time. When I worked in aerospace, stuff changed at least once a week. This forced me to come up with a way of documenting the hardware I was working on in a way that was easy to update. I also would document from 3 different approaches that each worked the best depending on what made the most sense when troubleshooting. That spilled over to how I managed the software on my computer. Since I was the lead avionics person, being the only avionics person, all of the e-CAD was on my computer along with licenses for Solidworks and other other SimWare. On my desk was a folder that contained all of the serial numbers, logons and passwords so if I were to get hit by a bus somebody could pull up whatever they needed that wasn't checked in at the time as a .pdf or other portable format. The folder also contained a whole bunch of other inane company procedure crap so it wasn't obvious that there was a page of credentials in there. Security wasn't a huge issue since there were only about 9 of us in the design office. When we had a proper engineering manager, he got a copy of my notes to keep and made everybody do the same thing. With just 9 people, nearly all of us were a one person departments. A big part of my job became documentation management since we started doing some government contract work and The Man is all about paper. Yes, it took away time from my working on electronics, but it also helped the company win a million dollar aerospace prize after we had a fire and had other people come in to help recover in one long night. A whole system was rewired with a single error while I slept.
I still got grief for spending time documenting stuff and trying to weed out the endless useless files in SVN that were automatically checked in since nobody ever received any training on which bits go in and which get tossed out.
If there is something in a company that is massively critical, it can't bottleneck through one person. Even worse if the person is a complete tosser. There has to be somebody in management that skips the 3 martini lunches and thinks about "What happens if this breaks?" What happens if the power goes out? What happens if this person leaves with no notice/dies/gets sick/defects to a competitor? If it's a janitor, easy, hire a new janitor. If it's an EVP, what procedure needs to be in place to do a secure exit process? If it's the guy in IT that has all the passwords, hire in a third party analyst to figure out what passwords that person better hand over to keep their job and good reference.