Because you hit the cryptic too soon, and you can't really avoid it. And I generalize this to any such language that has this characteristic, not just Scala. You can't use C++ for very long without having to know huge swathes of it if you want to use the libraries created by others, and you want to be able to debug why they don't work perfectly. (Bearing in mind "working perfectly" also includes you having a correct mental model of how they work, which is tricky if you only have a subset of the language in your head!) Same for Haskell. Compare with Python, where you basically can learn a reduced subset of the language, yet still use libraries fairly effectively. You need to know the basics of having objects and calling methods. You don't need to know how to write your own iterator, any of the double-underscore methods or the resulting protocols, you probably don't need to know decorators (and even if you do, probably just how to use them, because they are unlikely to bite you in odd ways if you just copy-paste instances of them), you don't need to know metaclasses, etc.
Type systems of almost any kind are very hard to satisfy without understanding. In some sense, this is one of the very constraints the stronger ones are trying to enforce.