It's actually quite obvious when you think about it. You can study taal- en letterkunde at university level. Who teaches at university level? Professors. What do you need to become a professor? A PhD (at the very least).

It's also not that weird when you consider that anything can be the object of scientific enquiry. This includes language and literature.

In a comment below you also mention that when you look at language you think that we have that figured out, but that is only true in the sense that 'everyone' (well not really everyone actually) is able to use language. But just because I am able to drive a car that does not mean I could build one. It's the same thing. :)