"Endgame" is the bane of MMOs across the piece. But it's an inevitable consequence of progression, when there's no way that progress can ever be lost or eroded. But nowadays, the only concept people have of what an RPG is tinvolves "getting to levelcap" with varying amounts of storyfluff surrounding the route to that anticipated nirvana. RPG no longer means what it used to. "Role" means "Tank, DPS or Healer": your "job" in the game. Story, however well-realised, is done and gone, the first time you do the last climactic cut scene. After that, you're just stirring pixels and goofing around with your pals for the most part.
So that's what people expect to be doing. The "level grind" is just a means to the end of "gearing up for endgame raids", and even if taken at a legible pace, will only occupy a small percentage of the career of a single character, since the story is finite, but the "end game experience" seems to be amenable to infinite repetitiveness. However, adding more numbers and shiny looks is far, far easier and cheaper for development houses to do than create comprehensively varied stories that will keep your paying customer satisfied for as long as the good old endgame hamster wheel seems to be able to for many. That effort would take the budget and resources of a major production studio, or several, of the kind which produce TV programming (but with a slightly different focus at the technical/performance end). And the demographic isn't large/wealthy enough to support that.
So what does it matter if there's a "go straight to the end game. Do not bother with the story. Collect one set of gear." option? So long as the story is there for those who are here for it, and there are enough of them for the producer to justify the expense of producing it to their CFO, the story will remain. So less "line in the sand" and more "cheerlead for the story", no?