Without a down, up is meaningless. Without failure, there is no success. How does one measure without a depth of perception? The point is that if you always get something of use, then what excitement is there in getting anything at all?
What excitement is there in getting an item that you know just because of its item type you will never ever use it?
You never know what you will get. You could get something amazing or you could get something completely useless.
For example, just the other day I looted a chest near the two carnival performers and it gave me a legendary sword named "Peter Lee's Sword". The previous day I was playing with a friend and looted that same chest only to get some vendor junk. Funny thing is, I had no characters that were setup for that, but you know what? I started working one of them specifically for that after getting such a prize.
The excitement is in that you may get something amazing, or... you may get something you have no use for. That is what makes it appealing. If you know you are always going to get something of relevant use, then what I experienced can't happen. I would have ended up with yet another item of relevance to my party.
Hey, to each their own though. If people want static loot, by all means wait for a mod where someone goes in and makes everything static or rewards people with a guarantee every time. Personally, I place such loot systems right up there with level scaling. /shrug