Speaking as a column manufacturer:
Pitch 'em. First of all, when one swages on an end fitting, the radial compression bulges out the frit sideways with such force that it distorts the end fitting (creating the leaktight seal in the process). That distortion is permanent, meaning that you'd have to seal it even tighter the next time to avoid having a gap between the side of the frit and the wall of the end fitting. The same may be true of the tube depending on how thick the wall is and what pressure was used during packing. Second, the cost of the components to a manufacturer is cheaper than the amount of labor and solvents necessary to clean them out thoroughly enough to make them reusable, and there's also the possibility of scoring the inner walls of the tube during packing or unpacking and cleaning. No reputable manufacturer reuses column components. Any company that offered a discount if you sent in a used column when buying a new one was just using that as a marketing ploy to get repeat business; the used column went into the trash.
