Why is Harrenhal still rich?

[Lord Vance II]

The familiar dichotomy of Harrenhal is of a massive, ruined, haunted, probably cursed castle that is still very desirable because of it's massive, fertile holdings. My question is why does a cursed castle that hasn't seen a particularly powerful Lord since Harren the Black, still have massive and rich land holdings? You'd think at some point since the conquest some Targ King might slice off a tract to reward someone for service. The way they talk about Harrenhal and it's holdings, it sounds like one could carve the land up between 3 holdings and they still be pretty nice. In Dunk & Egg we see land stripped from one house and given to another as a result of politics and war. I can't think of a particular person who deserved to be awarded a fat tract of Harren land, but I'm sure there has been. 

Why leave some of the most choice land in Westeros tied to a cursed castle that has an undeniable history of getting it's occupants killed?

[elder brother jonothor dar]

Well the lands and title have passed hands several time, the first few times there was no such thing as a curse, after that the reward of such plum lands and promotion was too attractive to pass up, finally we reach Littlefinger who looks on it from a practical point of view.

The castle too big the population decimated and lands pillaged, but the title and lands are still valuable.

 

There is no reason why a king could not split the lands if he so chose we do know that harrenhall was too grand for Slynts lowly station but a price worthy of LF for brining the Tyrells back and keeping the Vale in the fold.

[The Hoare]

Lords usually only lose part of their lands if they commit treason, but keep their heads intact. The lords of Harrenhall doesn't seem to be a treasonous bunch, or at least doesn't side with losers

The whole curse could be a myth. Who knows.

[cpg2016]

5 hours ago, Lord Vance II said:

The familiar dichotomy of Harrenhal is of a massive, ruined, haunted, probably cursed castle that is still very desirable because of it's massive, fertile holdings. My question is why does a cursed castle that hasn't seen a particularly powerful Lord since Harren the Black, still have massive and rich land holdings? You'd think at some point since the conquest some Targ King might slice off a tract to reward someone for service. The way they talk about Harrenhal and it's holdings, it sounds like one could carve the land up between 3 holdings and they still be pretty nice. In Dunk & Egg we see land stripped from one house and given to another as a result of politics and war. I can't think of a particular person who deserved to be awarded a fat tract of Harren land, but I'm sure there has been. 

Why leave some of the most choice land in Westeros tied to a cursed castle that has an undeniable history of getting it's occupants killed?

Because the demesne of Harrenhall is a royal fief and not a Riverlander one.  In other words, the Tully's hold nominal sway over Harrenhall, as it is in the Riverlands, but cannot name a new family to inherit it.  It is strongly implied by the text that Harrenhall is among the very richest and most powerful seats that aren't Lord Paramounts, perhaps THE most powerful.  For the Targaryens or Baratheons to slice off pieces of it might risk those pieces being absorbed into the Riverlands, and not able to be used as a reward for loyalties.  Remember, it is given as a reward for services to the Crown already.

Also, as others have pointed out, there is a feudal contract in which, theoretically, your liege can't just take your land for no reason.  If one of the various Houses holding Harrenhall had rebelled, I'm sure we'd see a different story.  Which is why I think the Targs didn't slice it into three or four powerful, but not hugely powerful, holdings.  It also probably serves as a great bastion of royal power in the middle of the continent and a way to project political influence and stability into the notoriously fractious Riverlands.  If the Bracken/Blackwood feud gets out of hand again, or someone rises up against the Tullys, the Targaryens have someone there who can step in quickly and even the balance of power.