Because it's original work they contributed for free. Lending others that kind of expertise and time, just that it get's used by a machine learning algorithm, which aims to reproduce this, without giving it back to them or the community in a similar free manner, feels violating.
Apart from that, creators feel ownership over their content and it feels wrong not to be asked what happens to it. (Although those probably wouldn't – or shouldn't – use SO anyway, as their content gets commercialised anyway by giving it SO for free.)

It gives worse answers and hallucinates a lot, problems specific to the model's design and not how much training data it gets. Even if it did work, it would be taking jobs away from actual programmers. It's a total net negative, users have nothing to benefit from this. Plus, since this is a community of programmers, they're all very much aware of these limitations and the lack of ethics of OpenAI.