This is The Nightmare. Devs building systems they barely understand, complexity leaking all over the place, and someone inheriting that awful job of keeping it running without having made any bad decisions themselves.

Software is full of these systems.

The more I learn about computers/software, the more I think it is a miracle that anything works at all.

There's a large financial incentive to making it work.

There's an infinitely smaller financial incentive to making sure it works well and functions securely. Thus it rarely happens...