Because it gets mentioned far too often for ideas to add to GalCiv2, despite the pointing out of the fact that the devs just don't have the time to both code the things in mechanically, and make sure the AI will make use of them... as well as for them to add any worthwhile gameplay. As it stands, I agree with the devs that, as GalCiv2 is, such a ship type would not be all that revolutionary.
And maybe, just maybe, you could see them whenever GalCiv3 is... but for now.. not happening unless someone manages to mod any one of the ideas put forth.
I personally like the 'Tanker' (aka Mobile Starbase) idea, where you can have a 'Tanker' class vessel that can extend ship range. It should not function on its own supplied range, and rely entirely on proper planets and starbases, for the sake of limiting them and keeping them from just traveling out into the middle of space with a fleet. They should just be 'relayers' of support/supply, not generators.
I suppose the AI would have some trouble making use of this, but it'd likely be easier than working with proper carriers.
