Hard dungeons are Different from hard raids due to this:
1. Dungeons require 5 people, Raids require many.
2. Dungeons are for the people who have little to no time to play, and yet still get to the ilvl of Normal/Heroic raiders. Meanwhile, mythic raiding is a race.
3. Dungeons get harder as time progresses, and when you get higher keys. Meanwhile, mythic is hard to be hard....