That C++ already has many implementations of sparse sets seems to be a point in favor of sparse sets rather than a point against Rust, especially given that C++ doesn't need them the same way Rust does.
Well, there's several implementations because it's a bit of a leaky abstraction, and implementation details matter/vary across use-cases, which is consistent w/ my experience of rust having heavy fragmentation in applying the array-index pattern for "weak pointers."