Because it's a loot based game. You complete the story campaign once which is boring for BL1 but quite awesome in BL2 then go farming. Most of the time, those farming hours are probably spent farming the legendaries and unique weapons.

U get real kickass weapons like this: http://www.youtube.com/watch?v=K_qyhMLMx5U Then you wanna go collecting them all. I once farmed 3 days straight after work on the same boss for a legendary shield that only dropped after 3 days.

I have 1000 hours on the X series. Screw Rebirth though, horrible "sequel".

I haven't went over 100hrs for any game, does that mean I have a life? :P

Nope, that means you hide the fact you don't from Gaben. :-P
Just kidding. That means you're all right (although I'd revoke your gamer card for that)

My top 5:

TES 5 Skyrim - 203 hours

Trials 2 Second Edition - 134 hours

TES 4 Oblivion - 126 (and counting)

Fallout New Vegas 120 hours

Just Cause 2 - 101 hours

My god, would they really take my card?! >_<

Skyrim at 198 hours. I think I have a problem, because I don't actually enjoy it all that much, as much as I enjoy just trying different mods

What if you have 1,000 games each played for 20 hours?!

I was wondering when someone would bring that up! That is more like me than 100 hrs haha.

Civilization V - 255h - still not tired of this game
Skyrim - 209h - i need to finish the dragonborn DLC
LOTRO - 113 - no idea when i put so many hours in this game but it is the last mmo i played
Borderlands 2 - 85h - whenever i feel like casual shooting i start it up
Euro Truck Simulator 2 - 74h - I'm thinking of restarting my company due to the new patches

Ahhhh, yeah, i understand. That's not for me, then. I hate grinding and i hate farming (which is why i hate most Korean MMOs). And if i do farm/grind, i'd rather do it in a MMO. In fact, this is my biggest complaint about Borderlands: it felt like i was playing a MMO in singleplayer. The gameplay structure itself is very similar to modern MMOs in my opinion.

Borderlands 2 117hr
Grim Dawn 95hr (despite being in EA - im a early alpha/beta KS backer)
Borderlands 1 46hr
Killing Floor 25hr
Mass Effect 17hr
Sleeping Dogs 10hr
Path of Exile 15hr
Football Manager 2013 23hr

NB: i have played several games with much longer hours before i had them on/ or still non steam Steam - older non steam Football Manager, Sacred 2,
NB2 : plenty of games only got like 1 or 2 hours or less since i tend to install & test. Thats it LOL.

Brother! Well, little brother at only 335 but I still love that you've been on the Floor a few times.

1. Killing Floor - 973 hours
2. Team Fortress 2 - 493 hours
3. CounterStrike:Source - 400+? I used to play this nonstop 24/7 for a couple years but it was back before they added gameplay hours.
4. Left 4 Dead 2 - 228 hours
5. Fallout: New Vegas - 138 honestly enjoyed hours. My favorite Obsidian game and favorite Bethesda game.
6. Left 4 Dead - 105 Hours
7. Mass Effect - 98+ hours. One of my favorite games of all time, easily. I have 74 hours on record because I had to play at least one time through with the Steam interface off because it lagged me so much back then.
8. Borderlands 2 - 87 hours and still haven't finished the story yet
9. Borderlands - 77 hours.
10. Saints Row: The Third - 75 hours.

Terraria - 110 hours
Orcs Must Die! 2 - 106 hours
Hinterland - 98 hours
Orcs Must Die! - 90 hours

There are plenty of games that handily beat these that I have not played through Steam. The Orcs Must Die! games are really good, but I am a little surprised that the other two are up there.

My link to it.. http://steamcommunity.com/id/kwama57/games?tab=all

Warframe close to 600, about 150 AFk or more perhaps.

Elder scrolls, Borderlands.. Saints Row 3, Dead Island.. Etc..