Dual layer discs are more expensive than HDD space. If you collect anything worth collecting, like DVDISOs, HDDs are cheaper.
Also factoring in all the wasted space since you can't burn to 100% of the capacity lowers the value of discs. I suppose you are a basement-dweller whose time is worthless but some people have better things to do than burn hundreds of discs. On top of that HDD space is reusable and can be easily freed up. DVDs on the other hand are disposable.
HDDs also win on convenience, ease of use, durability, longevity, organizability, and physical size.