I don't know where you people are getting that from.
I'm currently playing a thief/assassin that I created with the sole purpose of completing the Thieves Guild and DB later on. According to my save file I'm 20 hours in with this character and I still haven't completed the TG questline.
I'm not rushing the game but I'm using carriages whenever possible. I.E: I used a carriage to go from Riften to Solitude and from Windhelm to Markath, both travels would take a long time by foot.
I haven't done any other side quests, I even skipped Riverwood altogether after escaping Helgen. I don't use fast travel tho but even if I did it wouldn't have shortned much of this time.
So I wouldn't call 20+ hours short for a guild questline. Most singleplayer games don't have half of that game length in their main campaign.
Forcing them to last longer artificially inflates the time, yes. Doing the quests straight through makes them ridiculously short. Also, some of them end abruptly, and feel like there should be more.
it takes as long as you want it to mate, thats what our younger pairs don't understand or have the discipline to enforce

as for the DB, you are in for a treat if you can stomach the cold-bloodness of it
Has nothing to do with age. Thieves' guild feels a little longer. Companions and Winterhold feel stupid short. Yeah, I can artificially drag it out and inflate the amount of time it takes, but why bother?
Some are shorter than others, but alot of people just try and rush them them, instead of truly enjoying them.
I wasn't rushing. I simply played through the quests at an average pace. The only ones that felt like they lasted a decent amount of time were thieves' guild and dark brotherhood. And the DB only lasted as long as it did because I took the time to kill my targets strategically. I could've just killed them straight up, but killing them in interesting ways was more fun.
Given the fact the Companions is more of a social club, and the College of Winterhold is, you know, a place of education....why did they need a central story arc in the first place? :huh:
They're factions in the game, so yeah. They needed a story arc. They are both very short and very shallow. People can make all the excuses they like, but those two factions were pitiful.