I think it's a bit like with a language, that evolves as result of common use and compound effect of minor corrections and then we end up with something that works really well even though it could in places be engineered into something neater and simpler and uniform.

Main purpose of score is strictly practical - a performance and learning aid and I think to show which notes are consequently sharpened or flatten in a piece is all information performer needs (that's where the fingers should be ready to go). 

Also the fact that Key Signature is called that way might give impression that it's there to define the key of the tune. Well all it gives is a practical proxy to establish the key, and key of the tune actually can be sometimes up for a debate - it is not a definite information as opposed to definite information about sharps and flats. It's a bit of an abstraction on top of sharps and flats.

Finally one can call it ambiguity, and another - a very informative practical unification that tells us that from the performance point of view A minor C major and D dorian are essentially the same sets of notes