That’s probably because you can’t write a production-ready service or application from scratch over the course of an interview.
A violinist might be asked to play a technically difficult but short piece during an audition, even when most of the music they would play as a member of the orchestra would be less challenging and take a much longer time to play, with plenty of time for rehearsing ahead of time. Likewise, a software engineer is ideally asked to solve a small but technically challenging problem during the interview.
> A violinist might be asked to play a technically difficult but short piece during an audition,
No. Compared to what we do as software developers, the correct analogy for a violinist interview would be to ask them to whiteboard all kinds of obscure music theory principles, like the set theory underlying serialism and Arnold Schoenberg's twelve-tone technique.
A violinist might be asked to play a technically difficult but short piece during an audition, even when most of the music they would play as a member of the orchestra would be less challenging and take a much longer time to play, with plenty of time for rehearsing ahead of time. Likewise, a software engineer is ideally asked to solve a small but technically challenging problem during the interview.