Why don't graduate math texts have solutions to their exercises?
For graduate-level math books, the answer is typically not a value but a complete proof---typically of a related but relatively uninteresting topic.  For example, one of the first exercises in the Neukirch book you reference is:


  Show that the ring Z[i] cannot be ordered.


For such problems, the proof itself is typically much less educational than the process of struggling with the concepts in order to produce said proof.  

By the time that students are taking graduate-level mathematical courses, they are expected to have already mastered the general skills of constructing proofs.  Seeing how somebody else has proved a point is thus not expected to be particularly educational, whereas struggling to prove something oneself forces a student to engage deeply with the material at hand.

Finally, examples of working with the concepts in the exercises are typically already given in the chapter, in the proofs of the main results, so adding extra examples by working proofs for the exercises would typically be of only incremental benefit, but undermine the value of students having to work through the proofs themselves.