Because if you translate it literally into Chinese as "我們會永遠擁有巴黎", very few Chinese people will probably understand it correctly. They will think that it means "we will always possess/own Paris." Instead, this is a famous line from Casablanca, in which the protagonists Rick and Ilsa had a romantic time in Paris. This line therefore means that "we will always have the memories we shared in Paris," and as a result it is better to translate it the way you quoted, because that translation captures the real meaning of the shortened English quote, "We'll always have Paris."

Translator's prerogative? :) The sentence comes from the movie "Casablanca". In the past, Humphrey Bogart and Ingrid Bergman were lovers, and their happiest time together was when they were in Paris. Now the situation has changed (she's married and her allegiances are there, there are political considerations, etc.) and they can no longer be together. But, I think, Bogart says, "We'll always have Paris." by which he means that they will always have the pleasant memories of their time in Paris and that those feelings will not change. For someone else to quote this movie line means that although the present situation may have changed, the pleasant memories of the past and the feelings associated with them will *not* change, which in certain contexts could certainly be be translated as, "我们的情谊永在."