Well, Paris has possibly the world's most ideal geology for tunneling... soft limestone that is easy to cut, but is strong enough that it doesn't collapse during excavations.
Today, though, all tunneling is done with TBMs so geology doesn't really drive costs much. You just use a different kind of TBM.
I think France is just a country with an unusually strong pride in their public works. Not just transit - parks, public buildings, streetscapes, social housing, etc are all treated with a high level of design and unwavering commitment by politicians and the public.
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la forme d'une ville change plus vite, hélas! que le coeur d'un mortel...
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