Testing to ensure the product works as expected is more than half of the product development labor if you want a quality product. This includes time spends on things like the mandatory "anti-harassment" training any competent HR is forcing you to once in a while even though not related to product delivery (or so I hope - some should be fired for the problems you are causing by not living that training)

LLMs can write a lot of code. they can even write a comprehensive test suite for that code. However they can't tell you if it doesn't work because of some interaction with something else you didn't think about. They can't tell you that all race conditions are really fixed (despite being somewhat good at tracking them down when known). They can't tell you that the program doesn't work because it doesn't do something critical that nobody thought to write into the requirements until you noticed it was missing.