That's because Swift doesn't have a first-class concurrency story yet. I imagine that concurrency safety will be sorted out when Swift gets concurrency, but in the meantime all Swift concurrency is using C primitives like pthreads and libdispatch.
That sounds like it's going to be a mess. If they introduce compile time checks that have the same strictness as Rust it will break existing code. It seems like Swift has already done that a few times over.
Using a new language concurrency feature would require you to change your code anyway, the same way using any new feature would. And when the Elm/Rust-style ownership annotations are made public, they will be opt-in.
