It's partially because of that old saying: "Truth is stranger than fiction!" I think what drew me to my Medieval RPFs in particular was at first being into say, fantasy dynastic stories, but then when you learn about REAL dynastic families from the past, for me it was the Plantagenets, you realize what inspired the tropes that inspired the stories that are written today. Even in their own time, they captured the imagination of the people around them, who used the events of their lives to create moral narratives and connect to symbolism. I find there's a kind of specificity to real life history that you can't find in fiction, since irl things happen due to so many factors, including the factor of randomness, that is hard to get in fiction which must, by paradoxical nature, be a lot more structured and neat to be believable. In real life, everything takes place in the same cinematic Universe that you and I live in....
People's lives, especially public figures such as celebrities, politicians, monarchs, artists, etc, are fascinating to the general public and always have. Because they aren't just living their own lives, they also come to represent certain concepts or character traits that are beyond themselves as individuals. People can and WILL make up assumptions and stories about them in their own minds based on their own relationships, whether or not they admit to themselves that it is a fiction.
I think this is a very natural and human impulse but also I think somehow it becomes "weird" if you acknowledge it as fantasy and start working in that creative space as a self indulgence. There's a weird assumption that it's okay to make up wild stories in your head about public or famous figures if it's under the guise of a "joke" or "irony" or (God forbid) "political commentary," but if you acknowledge your active participation in it, it becomes awkward.
For me, I prefer Old Timey Medieval Historical RPF because well, it's more distanced from modern, alive people, but because also in a way it's more of a "challenge." For 12th century, it's a very unique era in that it's not quite "ancient" and yet it's right at the beginning of what maybe people culturally consider to be "medieval." It was a time that was legendary in its own time, and became the "medieval" legend even to people in later medieval centuries. Richard the Lionheart, King John, etc and other figures become known because of their roles in legends and stories rather than because of their historicity. And I find it neat to dig into the actual history and recorded facts of their existence and relationships because it's a bit of a demystifying exercise. There's both a lot more documentation than you would think, and yet there's a lot of gaps and distance that leaves a lot of room, in my opinion, for creativity. (And for inserting the YAOI delusions).
It also is nice because at times you feel like you're in conversation with the different historians or different sources you read about people, since oftentimes they do disagree and have their own agendas, so piecing together a type of character that feels true to you from that information is a neat project in and of itself. I've referred to it before as kind of putting together a "dolly" based on information that you can then use to play with in different scenarios, AUs, etc. everyone kind of has a different "dolly" since we kind of are drawn to different core features and aspects of characters and figures, and some specific things will be necessity need to be completely made up headcanons. But if we are all working from the same sources there will be significant overlap.
I like the challenge of trying to see how stories can work in this time frame, and this specific era and culture. And what aspects connect to me as a modern person as well as learning about the specific alienating parts of the era as well. Like another famous and much more talented RPFer before me (James Goldman of The Lion in Winter fame), sometimes the characters just start to feel very real even through that historical distance and you do feel a connection with them. Even if that connection is maybe more built upon a mix of historical evidence and your own tastes and preferences in fiction. It's all very fun!
I don't at this point (who knows lol. Never say Never) really have interest in like, current real life people like celebs and musicians or sportsmen etc because idk it feels too volatile to me and even without the RPF aspect Ive never enjoyed "stanning" people who are alive, or even from the last century.... I try not to think about their lives as much as I can since it feels too "close" in proximity. And also because this is an age of information, it's almost like we can know TOO much about each other--theres very little room for mystery and speculation if every interview, every photo, every social media post is available to consume I think. Also as a sicko who likes uneven evil power dynamics stuff, I guess id rather fantasize about it in ye olde times than in more modern times where it can get TOO real. Also because the power dynamics of older societies are fascinating to me in how they operate differently than now