If Rune Factory emphasizes the fantasy element and exploration too much, Farming Simulator does the exact opposite.
The series' focus shouldn't be too difficult to figure out, given its name. Farming Simulator is basically the agricultural version of Sim City or Cities: Skylines. You're in complete control of your farm, from purchasing plots and fields to buying machinery, keeping everything watered, and pretty much everything an actual farmer would do.
It's an impressive management and business game, and while the realism in the graphics isn't tremendously impressive, the more recent iterations certainly look good. Farming Simulator 19 even introduces John Deere machines for the first time, and you can't get more farm-y than that.
It also represents the main reason why it can't stack up to Stardew Valley. On its own, FS is fine, but there's no denying Stardew is a better game all around.
For one thing, FS is a rather lonely experience. Since the focus is farm management, you don't get the social aspect that's practically synonymous with "farming game" thanks to Harvest Moon. It's a business, so there's no place for getting attached to your animals either; they're just another asset to manage.
No magical or even monstrous small creatures to help make your farm a bright and lovely place either.
Stardew Valley respects the fact that people need to leave their farms and businesses every now and again, even if it's just to wander around in the forest — the forest that can't be bought and turned into another field.
Then there's the other problem of reality: it's too real. The appeal of managing one's own farm quickly withers when you live in a rural area. You see farms and John Deere when you leave the house, it's all anyone talks about, it's on the news every day, and unless you're really dedicated, it's the last thing you want to see when you jump into a game world.
Sometimes, a bit of fantasy isn't a bad thing.