I'm not convinced this actually happens. Seems more like somthing people assume happens because they don't like whatever codebase is at the new job.

If your TC is 500k-1M and you don’t feel like job hopping anymore, you’d certainly not want to get hit by a random layoff due to insufficient organizational masculinity or whatever. Maintaining a complex blob of mission critical code is one way of increasing your survival chances, though of course nothing is guaranteed.

The challenge is that sufficiently bad code could be intentional or it could be from a lack of skill.

For example, I've seen a C# application where every function takes in and outputs an array of objects, supposedly built that way so the internal code can be modified without ever having to worry about the contract breaking. It was just as bad as you are imagining, probably worse. Was that incompetence or building things to be so complicated that others would struggle to work on it?

Oh, I'm convinced, I've seen it first hand.