The filler problem is just part of a bigger problem.
A 10-12 hour Final Fantasy would flop too because people's expectations are for a lot more than that. So that's a dead end.
So they throw in padding/side quests because it's the main way you can extend the size of a game without increasing the cost exponentially. Unfortunately it creates bloat but you hit the requisite "30+ hours minimum" bullet point for your marketing.