It's a good old XY problem. Funnily I've heard customers that doesn't wanna bother getting asked why, they just want someone who do what they're told. Some junior engineers too, just wants the answer of what is asked instead of "the real problem".
First of all, it's often a false dichotomy: you can answer the X directly and dig deeper to find Y.
Second, it places the questioner in a "normal" bucket: surely they must have the same boring mundane needs as everyone else, and we just need to figure out which one it is (Y). But some people really are working on something interesting, and need to understand X to do so.
Thirdly, even if Y really is better for them, they still might benefit from an answer to X to avoid asking future variations of X.