Believe me or not, but yes, I am a devoted completionist and always do all the sidequests, explore all the zones, talk with all the characters, kill all the mobs I can find at least once and so on. That's how I manage to spend sometimes 120 hours on DAO with expansions and DLCs while most people are usually done in 60 hours or so. And, specifically for Mass Effect, for some reason I just adore exploring all the planets there, collecting all resources and such.
Yes, there is a lot of filler content in single player RPGs as well, as I stated in my post. However, there is two major differences:
1) The percentage of it in single player RPGs is generally much lower. Dragon Age: Origins surely has Fade, Deep Roads with a lot of Darkspawn just standing in there and waiting for you to attack them, Tower of Ishal with 4 levels, Circle of Magi with 5 levels... However, it also has hundreds of PCs having meaningful, fully voiced conversations. It also has many companions to talk to. I don't want to compare it now, but I don't think I would be far from truth if I said that 3/4 of the overall time spent in DAO was meaningful experience which I really enjoyed. In MMOs that percentage is totally different: even in SWTOR where it is said to be the focus of leveling ("story-driven", remember?), at least, 4 hours out of 5 were spent "killing 20 Imperials" and such and only a very tiny fraction of time I actually had some meaningful conversations expanding on the story and lore, some exploration of some important zones (instead of circling around some rock looking for 1 more crate to loot as needed in the quest). I don't even mention GW2 where the main story is not even that important at all and WoW with no character story whatsoever, only some quest lines related to races or factions as a whole.
2) Those fillers are usually somehow correlated with the story. For example, you mentioned the Deep Roads with hordes of Darkspawn, but they kind of make sense lore-wise, Deep Roads were said to have hordes of Darkspawn. Chantry Boards, Blackstone Irregulars and Mage's Collective Bag were lame, I agree, and they definitely were copy-pasted from MMOs, but there were very few such really pointless and lore-less quests. Now, look at any zone in WoW, say, Zangarmash (my favorite post-60 zone). You come to this completely new zone for you about which you don't know anything. You kind of expect to get some basics of the lore and history of the place, of the general problems and such. You expect to do something really important for the Cenarion Circle. Instead, you only get a bunch of weird quests, "Kill me 20 large flying things so I could have something to eat", "Collect me 20 samples of moss so I could analyze it", "Kill 15 nasty Nagas so we could walk around more freely". And my excitement about the new zone immediately vanishes, I realize that it is nothing different from Hellfire Peninsula I just came from, short of looks.