Because it is anchored in the Basic Law. To abolish the civil service, the Basic Law would have to be changed.
In the Basic Law: "The law governing the public service shall be regulated and further developed taking into account the traditional principles of the professional civil service." The word "further developed" allows a lot of leeway even without changing the Basic Law. Although I personally find it nonsensical, a further development could be that teachers are no longer appointed as civil servants. Just like the postmen back then. That is just a food for thought, without wanting to attack teachers now. Instead of teachers, one could also take other civil servant professions as an example. Even the police have been supplemented cost-effectively with the regulatory office. It is the task of politics to carry out exactly such changes, instead of always maintaining the status quo, because otherwise one could just abolish politics as well.