I feel qualified to offer a guess as to why: in Zig (and Odin) reuse is mainly done through what basically is templates. This reduces the need for vtables only when true ”plugin” type of objects are the only solution. For Zig and Odin, the only stdlib usages are for allocators and streams. These few usecases are way too few to motivate a full fledged interface feature, and consequently it’s not added. So it’s both a lack of usecases, as well as a desire to prefer templates over interfaces for reuse, typically due to performance concerns.