Because a curse isn't a pact, and an entity would have no reason to give power to someone bargaining at a disadvantage. Also using the example of fiends, they can't force someone down the path of damnation, they have to be willing participants. A fiend's job is to try and get you to ruin your soul without having to give you power. An idiot trying to deal with a devil will lose his soul and gain nothing but trouble.Why? I would allow that storyline, no problem. A family curse makes perfect sense for a warlock.
"A family curse. No problem, that's easy. But what I need you to do is...do you know that noble in the big house, he did a deal,with me some years ago. I need you to take this dagger, and kill his 3 year old daughter with it. It'll collect her soul, which is what he owes me. Once you bring me back the dagger, and a pint of her blood, I can remove this pesky curse."