I’ve spent many, many hours playing Sid Meier’s Pirates!, and there’s the same addictive quality about Mount & Blade: Warband. There’s always just one more thing you have to do, just turn in this quest, oh, but you ran into some bandits on the way, oh, and now your fiefdom is being raided, so you have to go defend it, but you need to recruit more troops, and sell off the loot you got from the bandits too. This constant flow of tasks has no easily-identifiable break points, so you never feel like you can leave it alone, even though the reality is you can save and exit at any time, and pick up exactly where you left off.
I think what really keeps you playing a game like this is that you might not remember what you were doing when you return to it later. Because almost all of what you do, and the order in which you do it, is entirely up to the player, you have to take on the whole cognitive load of planning and scheduling tasks. This is what takes up the bulk of the player’s attention while playing Mount & Blade: Warband, and once they’re in that state, there’s a certain inertia to it.
So it’s actually fortunate that you can’t play Mount & Blade: Warband for too long without running into one of the seemingly endless array of bugs, that snaps you out of that cognitive state, often by making one of your tasks impossible to complete. Those bugs are why I eventually had to just give up on the game altogether, because any given thing you’re doing might turn out to be utterly futile.