- Future proof. Unless I work for an outsourcing company, sooner or later I will want to push the envelope, or so I hope I do. And to push the envelop, one needs good CS fundamentals (maybe there are some exceptions in some specialized field). Think about React. It's a JS framework, yet to invent it one needs to understand at least compiler and graph.
- Geekiness/talent filter. The same reason that the nascent Google and Microsoft and any elite companies like Jane Street asked Putnam questions, Martin Gardner questions, ICPC questions, and clever probability puzzles. Whether it's a good idea is debatable, but at least those companies want to have such type of people and they were hugely successful. Note the word filter: people who pass the interview may not be good, but failing the interview means the candidate may not be smart or geeky enough. Again, I'm not endorsing the idea but just exploring the motivation of the interview policies.
- Information density. Assuming a company does want to time box a coding interview in an hour, it will be hard to come up with realistic question without spending too much time on the context. On the other hand, it's relatively easy to come up with a data structure/algorithm question that packs enough number of abstractions and inspection points to examine one's programming skill.
