There are HDDs of 200GB to 500GB size per unit and it's relatively easy to compress 8.5GB to a little over 4.7GB to 4.7GB or less, much easier at least than to compress DVD to DivX to fit 700MB CD.
It's easy to understand that there are people who probably feel it's worth spending a lot more on DL media if it's their jobs to distribute their own creations on write-once media, but what about the others? If one needs to distribute a lot of the same titles, "pre-recorded" DVD disks don't cost a lot, just as cheap as floppy diskettes and cheapest analog audio tapes.
I personally have terabytes of DL DVD and also have had ZIP drives and still have some ZIP media, but I only bought DL media for testing DL recording quality of new drives and firmware. (A little over 20 MKM001 2.4x DL disks.) I don't know anyone in South Korea who buys DL media for personal use.