I assume it comes down to they want to use the framework unchanged, the framework uses error codes rather than exceptions, so Swift would have to use them to call the framework even if it had exceptions.
A similar issue shows through when it comes to named parameters on the methods. Unlike other languages with named parameters you can't reorder the parameters when you call the methods, you really are calling an ObjC selector when it comes down to it and so the order is fixed.
CocoaTouch dodges all this stuff by layering its own C# flavour of the Cocoa framework on top of Cocoa. Clearly Apple took the approach of putting as little between Swift and Cocoa as possible.
I use C# and Javascript in a team, both of which have variables with no type defined in the code (when you opt to use var in C#, which we do, a lot). I can't say the lack of variable type visibility when merging has ever really being a problem. Either you know the code and so you know what it going on, or you don't and you probably shouldn't be the one merging it anyway...
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Remind me to change my signature to something more interesting someday