Unfortunately, the most business critical pieces of code tend to be the least regression tested. It's the earliest stuff that was made before any test frameworks were matured, it's been hacked on countless times by half the team based on shifting requirements to the point that no one understands it fully, and any future changes are such a high priority that it's "faster" to test it manually or in production.
I am of course guilty of that myself on some pieces of code. I try to prioritize cleaning up expensive tech debt, though. When folk are hesitant to modify a piece of code, it's a strong indication that the code is due for refactoring. It's always worth it as long as you implement regression testing in the process.
