I don't really agree that Golarion/FR/Midgard are "similar" per se. They represent three pretty different approaches to the same general idea, for my money, and Mystara represents a 4th.
The FR is very distinctive in it's own weird way, a messy, mystical, kind of cobweb-y setting full of unlikely well-meaning people, with a distinct lack of international conflict and war, and a lot of slightly LARP-y/ren-faire places in it, as well as a gigantic, endless pantheon which is weirdly the most real-seeming thing about the setting. There's stuff derived from real-world cultures, but it tends to be extremely superficial, like what hats they wear, rather than how they behave. It has a depth of content that certainly none of the others is close to, going back into the 1980s. There are multiple entire other settings inside it.
Midgard is a 1990s setting which resembles a specific type of '80s and '90s setting, which is to say the ones where most of the places are lightly based on Europe, Africa, the Near East and so on, with some fantasy nations scattered in-between. It has quite a lot more attitude than the FR, and a very distinct cosmology and being a two-sided "flat world" is kind of a big deal, too. It's generally a lot grimmer and darker than the FR, like it slid towards Warhammer, almost.
Golarion is a deeply 2000s setting, which is built pretty much entirely for and around adventuring, full of over-the-top, ultra-stereotyped places for adventurers to come from, and extremely dangerous places for them to go. It's extremely functional, top to bottom. There's little sense of it being "lived in" unlike the FR or Midgard, and geopolitics make no sense, but that's something they do all share (unlike Eberron). If you want a setting that feels real, it's not the way to go, but if you want one that's easy to understand and where everything is pretty clear, and there's room for gunslingers next to medieval knights, it's a good one.
Mystara is a peculiar setting. It's incredibly diverse. Really everything is somewhere in Mystara, in a neat little nation/kingdom situation right next to a totally different and unrelated nation/kingdom. It's like it was created by laying down hex tiles. You got irradiated elves, you got noble lion-men, you got native americans, you got germanic knights, and so on, all just sort of jammed together. Then you've got a whole region which is basically "the 1700s" for you to and mess around in (including guns). And you've got the whole hollow earth deal to boot. It doesn't make a lot of sense, and it's not quite as designed for practicality as Golarion, but it has a certain naive charm.
