Dinh AaronMk said
I don't know why it's not sought after, to be honest. If you just go as far as a map and some world details you could ideally have on it whatever you want and the lore becomes really more of a community thing, or in the very early stages so unwritten and unexplored nothing's really invalid to the story. It's only when things get so large or people set up so close that they need to perhaps know enough of each other's nation OOC that they can have consistent cultures or cultural story telling.It's so unburdened from the rigors of having to know about RL history to make an acceptable Alternate History RPs it's shocking it's ignored. And if you wanted medieval England, you could theoretically replicate that in such a setting.
Precisely why I love made up settings though strangely you will find few players, at least in my experience, willing to pitch in their own ideas for a fantasy setting. Which is Absolutely maddening when I think about it, who wouldn't want to help create a world and have a hand in it's creation? Knowing you helped create a interesting setting.
I've opened up co-world building in the past and...got very little interest, people tend to gravitate to A, worlds with per-established history and rules, or B, worlds that are already very detailed and leave little room for in put from the player. This has always just boggled my mind, but I suppose not many Roleplayers love world building as much as I do. I remember first stumbling across the Guild and the first Rp I saw was a NRP where players were literally mini-gods creating their own Race, cultures, histories etc. I loved the idea.
I think my NRP I made very recently is the only Fantasy NRP not based on pre-made setting(unless you count the Steam punk RP's). I guess people are not bored with WWI-II yet. I suppose people will always gravitate to what is familiar.