> It seems the candidates mostly have experience with loading data from one data source, doing some transformation with no regard for performance, calling APIs, and storing data into another data source.
You may not like it, but for the vast majority of programming jobs globally, that is pretty much the whole job.
Let me add a note for for you - the onus is on you find if someone can learn the task. That is what is a great interviewer does.
As masterj pointed out, a really large portion of jobs ( more than three forth) out there won't be nearly anywhere near the complexity of the task you mentioned.
You may not like it, but for the vast majority of programming jobs globally, that is pretty much the whole job.