What is going on right now?

functional code because it intentionally always takes input and returns output. However not all code is functional and testing has a side effect of making change harder. So if you write a lot of half-useless tests that break when you change anything then you've just made your code harder to change. Even with an AI doing that automatically the damage is contextual, which tests should be removed after a given change and which kept? It requires a decent amount of thought.

Outside of functional code, there's a lot out there which requires mutable state. This is much harder to test which is why user interface testing on native apps is always more painful and most people still run manual QA or use an entirely different testing approach.