Except so many of those pieces are not generic. There's dozens of color and shape specific pieces in there. Things that will never go on anything else.
That said, you can still buy generic lego boxes.
That said, you can still buy generic lego boxes.
e.g. The Star Destroyer engines are actually wheels presumably borrowed from some other set. The Death Star as far as I can tell has no "special" pieces, nor the AT-AT, AT-ST or X-wing. The millenium falcon has a few special bits, but even they look repurposed from some other set.
That you think these pieces will never go with anything else shows you aren't thinking about what you can do with them, only what they are intended to be.
Thats the beauty of lego and imagination. The piece can be anything you want it to be. Is it the exhaust of a spaceship or the ear of an alien space monster? You decide!