Some times series are more predictable than others. Being good at predicting the predictable ones is useful.

For example you can easily predict the weather with descent accuracy. Tomorrow is going to be about the same than today. From there you can work on better models.

Or predicting a failure in a factory because a vibration pattern on an industrial machine always ended up in a massive failure after a few days.

But I agree that if a model is good at predicting the stock market, it’s not going to be released.