Why vampirism can be cured: Magic.
No really. In a fantasy setting you can really just make up whatever you want to and chalk it up to being some old ancient forgotten god or whatever and be done with it. It'd be better if there was quality writing behind it but I mean really, it's magic. You don't really have to explain anything. But ghoulification is a mutation in Fallout. Everything should have a decent explanation for how it works and why and once you've written yourself into a corner there isn't a magic shortcut. SCIENCE! is not magic. SCIENCE! is simply used whenever something is literally scientifically impossible to make it possible and even then it should be used very very carefully as if you don't you risk whatever you're creating becoming absolutely absurd and ridiculous. And Fallout was never meant to be absolutely absurd and ridiculous. It had humor, sure and even the occasional oddity and absurdity but it was space out and it was often very very minor things.
Point I'm trying to make is that SCIENCE! is not magic. You can't use SCIENCE! as an excuse for everything. There needs to be a plausible explanation for why things work and SCIENCE! should be avoided at all costs, it should be indulged just to make whatever they want to add into the game a possibility. And even then, SCIENCE! needs to still be explained. With magic, you don't have to explain everything. You can simply go "this magic's real" and that's that. Why hasn't that magic been seen before? Oh it was lost, or it was ancient or you are someone like the Dragonborn and can utilize this magic through your specific bloodline. But you can't just shove in SCIENCE! wherever you please and think that's enough.
Is a cure feasible? Sure. With good enough writing it could be possible. But why would the player be so special as to attain it in the first place? I mean, it's [censored] writing IMO when plot convenience is used too often and the player character gets way too much of that as it is. Being able to turn into a ghoul and then being in the region where a cure for ghouls is under way is far too convenient and reeks of bad writing to me. Why would it come without any drawbacks? Cause we're talking about the equivalent to a cure to vampirism which means it's removed entirely, bonuses as well as drawbacks so you're back to being a human. Why would the people working on it work on it at all? With all the things that scientific communities would strive towards why would they bother with creating a cure for a set of mutants most of them despise? I mean, most ghoul communities we've seen aren't exactly exploring science. So it'd have to be a human community. And, well, why would they bother? Followers Of The Apocalypse might try to do it but they aren't exactly high-tech or anything. So who would do it? Why would they do it? How could they do it? And why should the player even have access to it in the first place?
In a fantasy game you can use any kind of cop out excuse you want to and players don't really have to question the validity of it all too much but this isn't a fantasy game. It's a post-apocalyptic game that works within its own setting's idea of "science".
Can it be done? Most likely, yeah. Hell, a giant robot scorpion is a thing in Fallout. Should it be done? No. Because a giant robot scorpion is a thing in Fallout. Fallout is not TES. If you want to become a ghoul then you are then a ghoul. There is no easy way out. You're stuck. You're a ghoul. Don't like it? Then don't become a ghoul in the first place. The player shouldn't be given anything (s)he asks for, and if the player doesn't like it then it is up to him/her to accept that you don't always get what you want. Ghoulification is not vampirism and should not be treated as an equivalent to it.
Remember that Fallout 3 was called Oblivion With Guns.