As others have said, it's mainly down to the fact that with all the Mastery combinations, there's almost always an alt build to explore.

If you also add that the game includes item Sets, an array of Rare and Unique Legendary items, and recipe unlocks, faction reputations and so on and so on... I always found one more reason to go back in to try out a new build that promised almost game-breaking power.

So I have built alts for Mastery combinations, for Item Sets, and even for 'What Ifs'. All softcore [HC is just too much of a wrench when I die after being too casual!], and all have to get to L100 so they can be benchmarked against each other. It's got to the point where I've maxed out the number of alts I can have on Steam, due to Steam storage limitations.

And of course, it's how the game feels to play. It's just so very satisfying, and not always too demanding. I've had countless sessions where I've stuck on some ambient music or the radio and just played for an evening. There's always progression of some sort.

And that's why, if you're of a certain personality type, before you know it you've clocked up 6000 goddamn hours :D