
It's the same reason why Diablo doesn't seem as weird today as maybe it should. In 1996 it wasn't as curious a design decision for Blizzard to build a supermodel-level game entirely on repetitive and reductionist combat. The game is a point-and-click Space Invaders, with attack, parry, thrust, defend, shoot, reload, zoom, alt fire, holster weapon, duck, strafe, dodge, walk, talk, blink, breathe all mapped to the left mouse button. Click click click on lots of monsters, and that's Diablo; that's carpal tunnel syndrome. Combat like this would be a relic in any game on a similar production scale, if not for that once upon a time Diablo was very successful and spawned a franchise and imitators based on that singular mechanic. Today, amongst Gears and Gods of War; varied and multi-layered combat systems that provide the player with an ever-expanding array of input and feedback options, evidently there's still a place for Diablo.
Diablo has fans, Blizzard has their money, and thus Diablo 3 can be absurdly simple to play. If another major title, carrying with it a level of hype comparable to Diablo 3, came out and it was click click click? Disaster. Diablo has nostalgia, and it has a pass. Diablo-clones can get a pass too, but they'll never be the real thing. Titan Quest and Hellgate: London never generated as much affection as Diablo 2 and as much enthusiasm as Diablo 3. They're merely filling a void, and now the dark lord has returned to claim his throne. Nobody does it half as good as him.
It has an exemption, and so it is to gameplay what Metal Gear Solid is to narrative. More accurately, Metal Gear Solid is to cutscenes what Diablo is to monotony.
Metal Gear grew up in the pre-Half-Life era of high cutscene tolerance. Now, they're in the process of being eradicated completely. Tearing down Metal Gear Solid for its cutscene length is a tired grievance these days, and in leveraging the "genius" of Hideo Kojima, it's an easily deflected critique.
Still, put anyone with a broad gaming literacy in front of Metal Gear Solid for the first time and they'll be stunned. They'll have the same reaction when they see Diablo. There's so many mouse clicks. There's so many cutscenes. These games were designed in a vacuum where contemporary design sensibility never applied. How did they get away with this? How are they still doing this?

Makes you feel sad for the rest. No one else can do what Kojima or Blizzard does, and what those two are doing is actually easier. Diablo's combat is as elementary as it gets. CliffyB can have all the paintball battlefield inspiration he wants, but Diablo, the anachronism machine, will remain a strong competitor. Meanwhile, other developers -- Valve, 2K -- are just as interested in telling a story as Kojima is, but without a history of lengthy cutscenes, they're stuck operating within modern narrative structures and gamer preferences. The preferences which say gamers don't care about cutscenes unless they're Hideo Kojima's. Marc Laidlaw and Ken Levine have to puzzle out a way to tell a story that doesn't wrench control from the player. It's easier to write a story as a screenplay than as audio fragments scattered around the architecture of a first-person-shooter. Kojima takes full advantage of his position; augmenting his epic saga with all the pseudo-science footnotes he wants. Like many have said, Kojima could use an editor, but no one's going to make him get one. For a writer like Kojima, the easiest setting is verbosity. Such is Kojima's luck that he gets to do what's easy.
Does it irritate the competition? Cover systems and squad AI can take months of work but click click click is a guaranteed hit? Perhaps it does. But when you look at why Kojima and Diablo are able to subsist at their most comfortable, it's because they never failed. They have the right to be nonconformists but they don't use it to make bad games. They never lost their audience. Never lost the critics, never lost the money, and never lost the right to ignore anyone who told them "no".
Kojima tells engrossing, emotional tales even though they're bizarre melodrama. It might be an unfairly discriminating set of circumstances that let him do so, but at that kind of intricate saga, he's the very best. Diablo's genius lies in its simplicity, as it translates to just one more monster addiction. They don't need to modernise it because everyone's already hooked. They trade on nostalgia, sure. But they'll never, ever betray those memories. They'll never stop reminding you of what you like about them. Diablo can stay conservative and Kojima can stay insane and they'll keep you coming home. Why?
Nobody does it better.