Mainly because they made one of the major selling points that the models were in scale relative to each other. Look at how much you're questioning the scale of the GR-75, which is far less of an adjustment than any of the Star Destroyers would require to be on the table. The Interdictor is at the small end of the "Imperial"-style ships and it still clocks in at 4 times the length of the corvette. Given that emphasis, they've already had to deal with a minor uproar at the announcement that the corvette and transport would deviate from 1/270 scale that the fighters have. It would be a huge outcry if they somehow altered the scale enough to fit a ship 10 times bigger than that onto the table. Looking cool isn't a good enough reason to absolutely wreck something
FFG has put that much effort into maintaining.
Then you also have to ask what would it do in the game? This is scale of a different sort, but still a scale. The Imperial Star Destroyer's fighter complement would translate, on its own, to in excess of 1000 points using the cheapest pilots available. That's about ten times the size of a normal X-Wing game at this point. This ignores the Star Destroyer's own hull and weaponry, which are quite formidable to say the least. So, how could that be balanced in the game? In reality, it can't because the Star Destroyer really doesn't have a place in X-Wing. X-Wing is a game about fighter combat, and the Star Destroyer is a carrier. There's a divide there that just can't be (reasonably) crossed, and
FFG seems to be smart enough to know it.
On the other hand,
FFG has all but come out and said that a Star Destroyer is being worked on. Given the number of capital ships shown in the movies, plus those added in the video games by LucasArts, they likely have enough ships to do a battleship-scale game. This is a vastly superior option even ignoring that it'd be a separate product with it's own revenue that could be properly balanced on its own.
FFG then doesn't have to betray the scale of the X-Wing game in a tournament setting and upset that segment of their customer base for pointless fanservice, but if players at a club want to pick up a Star Destroyer model from this other game and house-rule into it X-Wing...hey, it's their model. They can use it how they want on their own.
As far as the GR-75 goes, the pod could and likely is just the pilot's seat similar to the Millennium Falcon. The size of the pod looks wrong because of the scale adjustment to get it onto the table; it's not the same 1/270 scale as the X-Wing and other fighters (see also: why a Star Destroyer would look bad). That said, the ship itself is really just a shell that holds cargo containers via force field and then has a bunch of engines. It only has a crew of 7, which for a ship 7 times the length of an X-Wing and noticeably bigger in all other dimensions is about what you'd expect. More information on the GR-75 can be found here:
http://starwars.wikia.com/wiki/GR-75_medium_transport