Zero values prioritize implementation convenience (we always have a zero value so we don't need to handle any cases where we don't have a value, just says those are zero) over application convenience (maybe my type should not have a default and the situation where it has no value is an error)

Take either of Rust's library types named Ordering - core::cmp::Ordering (is five less than eight or greater?) or core::sync::atomic::Ordering (even if it's on another core this decrement definitely happens before that check) neither of these implements Default because even though internally they both use the zero bit pattern to mean something, that's a specific value, not a default.