And that's the problem. Because they are educational. And because they are both educational and fun, people are much more engaged to learn from them than from "real" educational games.
People who play Europe Universalis learn way more about early modern history than they ever learned at school. As Randal Munroe pointed out, you learn way more about orbital mechanics from KSP.
If you want to make truly educational games, you shouldn't focus on the educational part, but take the educational part and wrap it in tons of fun.
Of course there are topics where this is going to be hard. I have absolutely no idea how anyone could make a fun game that revolves around German grammar. (Or do I? The best way to learn a language always seems to be to actually use it with a native speaker. Having a friend who speaks the language you're trying to learn would be a great way to do that. There might be something here.)
But something like geology could be part of a simulation where you need to find certain resources, and that's easier once you understand how those resources are formed. And then there needs to be something fun to do with those resources, of course.