It wasn't possible until Swift 5 because of ABI stability. If we were to do that before Swift 5, we would still have to create binaries for each version of Xcode/iOS.
I understand it might sound weird to be telling people we're hiring and at the same time telling them that we can't use Swift, but we do use Swift for all new UI and unit tests we write, since those don't affect the framework. Plus, Swift aside, working on software that runs inside other developers' software has so many challenges that I'm sure are still going to be interesting regardless of the language you use.