Because any or all sorts of co-op experiences must pan out exactly like this one?
Actually, it isn't all that bad, if you're capable of tanking the level by yourself and don't mind your "party" changing every 5 minutes. Loot drops are player-specific, so any treasure on the ground is yours. The other players won't be able to see your loot, and you won't be able to see theirs, but it's pretty obvious when characters are hanging back during combat and then scooping up the treasure from other people's kills. They still count toward combat scaling. Before they introduced scaled Monster Power, or whatever they called it, that was actually a really good way to increase the difficulty level.
But Diablo III is pure hack-and-slash, with exactly the same story repeated 5 times as your character transitions to higher levels. There's dialogue, but it's pure auto-dialogue with no player input. That isn't what Dragon Age is about. Sure, they could introduce some kind of persistent world setup for multi-play like that, and it would be fun for occasional bouts of mayhem, but that niche is filled by other games.
Baldur's Gate/Baldur's Gate II had co-op multiplayer where player characters could take the place of followers. That was my introduction to video gaming, in fact. Fiance (merely boyfriend at the time) wanted somebody else to play Baldur's Gate with him, and I was getting tired of watching him bash monsters for hours at a time without having any clear idea what he was doing. It was OK, but it wasn't much fun watching the story revolve around his character, the Bhaalspawn, while my character was never even mentioned. I had very little investment in the game. And then I started my own game and discovered that there were actual followers with personalities and banter and romances, and that was the last time we played in co-op mode. We still play Diablo II and Diablo III together, but when it's a story-heavy game like Baldur's Gate II or Dragon Age, I'd rather stick to the single-player experience.
TL; DR: Co-op works great for hack-and-slash games, but it's less satisfying in story-rich games with a single protagonist supported by a cast of followers.