Jamin Warren talked about this a few years ago.
I think it's a mixture of being jaded after playing so many games and lack of time. I'm in the last year of my 20s and I don't think I'll ever play a game like Dragon Age: Inquisition again. I'm questioning if I should even bother finishing it. By my estimation, I'm only about 60-70% way through the content even though I've sunk 60 hours into the game.
The problem is a lot of the content is frustrating. The horse-riding is awful. The inventory and crafting systems are a mess. Every boss encounter feels the same. The difficulty scales badly -- the beginning is way too hard because you only have a handful of abilities. By the end, things become too easy, so you have to ramp it up. The war board is a totally unnecessary time sink and there's no easy way to tell which ones are worth pursuing (ones that will lead to quests or fulfilling quests rather than just resource gathering).
In my teens and early 20s, I didn't mind dealing with stuff like this because I had so much time. But now? I could be spending my leisure time doing things a lot more fulfilling than farming for herbs in a single-player game. I've done too much of these grinds to still feel excited about them.
Which is why Mass Effect 2 was such a phenomenal RPG to me. It cut out all the BS. No grinding, gathering is kept to a minimum, and all your missions were tied to progressing the story somehow. It wasn't a short game and everything in it was meaningful content. And no inventory system! Everyone cried bloody murder when Bioware removed the inventory system but think of how much time you waste killing bad guys and then shuffling around collecting every damn thing on their bodies.
More game designers need to learn from Mass Effect and the Arkham series. Arkham did collectible farming perfectly. Exploration was so satisfying in that game. Gliding around was fun and you had to use a variety of tools and exercise your wits to get them. Gathering each collectible was like a puzzle, and moreover, it didn't feel like pointless farming because all the collectibles led to story progression and quests. When I was grabbing Riddler trophies, I wasn't just doing them for an achievement or to unlock an item. I was literally hunting down the Riddler. I was unlocking dungeons to save cops from. It led to enjoyable content.
The filler design we so often see is just the modern version of fake difficulty. It's a cheap, thoughtless way to pad in gameplay hours. Now that I'm older, I see it for what it is and no longer have a high tolerance for it.