Why do LLMs do it for real: because you trained them by stealing all of Stack Overflow?
Less sarcastically but equally as true: they've learned from the tests you stole from people on the internet as well as the code you stole from people on the internet.
Most developers write tests for the wrong things, and many developers write tests that contain some bullshit edge case that they've been told to test (automatically to meet some coverage metric, or by a "senior" developer who got Dilbert principled away from the coalface and doesn't understand diminishing returns).
But then the end goal is to turn out code about as good as the average developer so they can be replaced more cheaply, so your LLM is meeting its objectives. Congrats.
Less sarcastically but equally as true: they've learned from the tests you stole from people on the internet as well as the code you stole from people on the internet.
Most developers write tests for the wrong things, and many developers write tests that contain some bullshit edge case that they've been told to test (automatically to meet some coverage metric, or by a "senior" developer who got Dilbert principled away from the coalface and doesn't understand diminishing returns).
But then the end goal is to turn out code about as good as the average developer so they can be replaced more cheaply, so your LLM is meeting its objectives. Congrats.