Because it describes the style of play that will get you through the game, rather than an obtuse difficulty with no measure.
Theoretically you should be able to make it through a 'casual' difficulty game playing like a casual - just jump in, spend 5 minutes to half an hour, head off. You don't need to put much thought into it, or a lot of practice, its just something you jump in every now and then to do for a laugh.
Normal is where an average player would be required to beat the game. You need to put some thought into your actions, and have developed some level of skill to be able to beat the game. You should be spending longer periods of time than a 'casual' difficulty run through in beating each mission, and thus would require longer sittings, though not always substantially, than a normal player.
Hardcore mode is for when a hardcore mentality is required to beat the game. You need to think about all your actions, you need to have honed your skills to a semi-competitive level, and you need a significant time investment to beat each mission because of this. After a lot of practice at that type of game, hardcore can just become second nature and be as easy as casual for some people, but that all depends on the game and the player.
Why this over Easy, Normal, Hard?
Well, I have no clue whether Dark Souls has an 'Easy' mode or not, but if it did, it certainly wouldn't be the type of easy where someone could jump in, play for a few minutes with little skill or thought, then jump out and eventually finish the game - at least if what I've been hearing about it is correct.
Hence, what meaning does "Easy" have when you first load up the game? You only have other games as benchmarks, and they provide an answer inconsistent with what the difficulty actually is. Such games do exist, where they are not meant to be played in a casual way, and 'easy' is merely a way of saying 'Not as hard as normal' - requiring you to have played the game before to be able to make a worthwhile comparison.
Casual, on the other hand, tells you everything you need to know about how to play that difficulty. Jump in, no thought, no skill, and eventually finish the game.
No clue how this holds up in practice, and I am obviously biased by my experience of difficulty levels personally and am finding it harder and harder to empathise with people who play on easy now, as I haven't had to play a game on that difficulty for years [Though I still turn them on easy sometimes for a casual run to waste time], so take this all with a grain of salt - but this is how it appears to me.