This. At cost-per-gigabyte, the only thing that might beat an optical disc is a hard drive (the old fashioned kind, NOT an SSD). Optical discs are also more reliable than flash drives. Flash drives have chips and firmware on them that can fail. Optical discs don't. Flash drives are not replacing optical drives anytime soon.
Even hard drives, it's fine if you're only comparing on a blank-to-blank media but add in the duplication costs to sell content on physical media and optical wins all the way against anything right now.