Except the entire design of swift is meant to make everything more automated.

* automated exclusivity with value types and value witness tables, classes as arc types (ie Arc<Mutex<T>>)

* automated interop with C/C++/Obj-C through the clang ast importer

Maybe they could have started with rust and added on what they needed, but why not build a new language at that point where things so fundamental are involved?

Source: I worked in lattners org at the time of swifts inception (on an unrelated backend) but that was the motivation. I also worked on the swift compiler for a little bit some years later on in my career.