Why does Rust not have a standard async runtime?

Before Rust, I've used languages that have async capabilities as a part of whatever the runtime it's running in (.NET, NodeJS, Python, Dart). It's the first time that I've used a language that relies on third parties to provide an async runtime. I understand that async runtimes are separated so that it doesn't bloat the resulting binary if you don't need the feature but why does it not have a standard runtime even if you have to install it separately?