What are some valid reasons why people wouldn't now use these rust libraries and extend them to their preferred language? Maintaining clients is tedious work and prone to abandonment.
I would expect AWS to provide custom libraries for basically every language. The cost of a few full-time engineers who are experts at any reasonably popular language is probably pocket change compared to how much even a few companies using that language might spend on AWS services.
Not all languages have a great interop story with Rust. Binding the JNI is especially tricky, for example. Furthermore, when performance isn't important, the need to package and compile Rust code may be an unnecessary hassle.