There has actually been an SDK available since the SYNC 3 days. I've played with it off & on, but it's not the easiest thing to work with, and even the sample apps that are provided with the SmartDeviceLink SDK don't connect 100% of the time. I also suspect that once CarPlay gets involved, things get a bit more complicated making sure that all of the correct bits are talking to each other. (And let's be honest, the Legal dept. is going to have their say, dictating when certain buttons are available, or what features can be used when, further complicating things. Especially when it's a mix of Ford legal and Apple's CarPlay restrictions.)
But if you're curious:
Honestly, it's a big UX problem too - even with the app attempting to walk you through the setup, it was never easy. Having to bounce between the SYNC UI to launch the app and then back into CarPlay isn't a great experience. Until they can find a way to just plug in your phone, launch the app once, and go, not many folks are going to want to put up with the app, especially if it often drops the connection to SYNC and needs to be reconnected.