Interview coding questions aren't like the day-to-day job, because of the nature of an interview.
In an hour-long interview, I have to be able to state the problem in a way the candidate can understand, within 10 minutes or so. We don't have time for a lecture on the intricacies of voucher calculation and global sales tax law.
It also has to be a problem that's solvable within about 40 minutes.
The problem needs to test the candidate meets the company's hiring bar - while also having enough nuance that there's an opportunity for absolutely great candidates to impress me.
And the problem has to be possible to state unambiguously. Can't have a candidate solving the problem, but failing the interview because there was a secret requirement and they failed to read my mind.
And of course, if we're doing it in person on a whiteboard (do people do that these days?) it has to be solvable without any reference to documentation.