Because it is the 2nd easiest possible data structure to implement, if you can't do that it means you don't grasp how data can point to other data and how you c

Because it is the 2nd easiest possible data structure to implement, if you can't do that it means you don't grasp how data can point to other data and how you can leverage recursion to make hard problems easy to solve. Perhaps they were hiring for someone that they needed to be capable of more than just trivial CRUD wiring. Even when you aren't writing core libraries these things come up constantly, sometimes at higher levels when designing systems and how they interact even. I implore you get to a state where this isn't hard for you, it will make you capable of more.