If that's true those employees could be kidnapped for a high ransom...

You would need most of the people. One high level person is meaningless. Sure you can say "Dave" knows everything, but any smart company knows Dave could retire, or die at anytime and so the important stuff he knows is spread among a bunch of other people. Often it is also written down, just not in an easy format.

Institutional knowledge is more akin to the ship of Theseus. Good companies know this, after all, even super knowledgeable employees sometimes look the wrong way when crossing the street.

That's a much higher bar than hacking into some computers. And also, as people pointed, unless you are wholesale kidnapping enough people to fill a prison, it doesn't work like that.