So what's the problem? Your interview process.
Considering that, of course the interview won’t be a slice of real work - there just isn’t enough time. We try to compress the software development experience of months to an hour - so it gets lossy. That’s why, in the interview, we gloss over details that we won’t in real life.
Which is where ChatGPT’s answer really excel. Sure, it won’t really be able to design and code the system. But it can mimic knowing how to do it for the purpose of the test quite well.
Why not? Why does a machine being able to pass a test take away from the usefulness of the test when applied to a human?
But presumably the op thinks the machine is not good enough to replace a developer. But it passed the interview... so doesn't that mean the interview question was bad?
No, it doesn't. You're assuming that the test and the job are the same thing which they don't have to be. My point is that you can have tests that are different from the job itself which are still useful tests. Then you don't care whether a large language model can pass the test too.