They don't, but C++ programmers especially are familiar with not letting inappropriate things escape scopes; they're much less familiar with not letting inappropriate things escape _expressions_. I've reviewed over a half million lines of code from engineers of every level of experience, and I've seen far more bugs from the latter than the former. For instance, a former employer had a function similar to folly::get_default, except it returned the default by reference, and engineers were always running afoul of lifetimes using that function. (It was especially egregious with std::string temporaries returned from string literal defaults.)

Thread annotations also effectively prevent unsynchronized access to the object, but they do it in a way that programmers make fewer mistakes with, and they do it with a less error prone mental model: mutexes are seen as guards of _class invariants_, not merely as guards of _data members_. Apart from encouraging a superior mental model, thread annotations also encourage a one-mutex-per-class model which I've found far more successful and error prone (both in code I've reviewed and in code I've written) over a one-mutex-per-member model which inevitably (in my experience) results in fewer bugs.