Because in Germany/Europe discrimination is officially illegal so it is assumed nobody does it, even though people know better, and because you cannot fully outlaw and police tribalism, human bias and cronyism that is evolutionary ingrained in us as a species since the dawn of man.
Putting more legislative hoops in place means HR will just need to find more creative ways to formulate job requirements in order to exclude certain classes of people or more creative ways of justifying not taking you application further, but ultimately won't solve the core issue.
There's also the Arbeitszeugnis thing (a summary of how they see you as an employee, given when you leave the company). If your boss doesn't like you, they can fuck up your future a bit by giving you a bad Zeugnis (future employers typically want to see those).
It's effectively illegal to complain about employees in Zeugnis, but they can reorder words and skip some phrases to imply that you're a bad employee. If you're not German you might even think you got a good Zeugnis because it looks well on the surface, but there's the entire code you need to learn to read these.
Sackings? On what ground? Proving there's any kind of malice or bias in rejected applications is nearly impossible as the main job of HR is to protect the company from any hiring bias leaking out. That's why your rejection email is usually some generic copy-paste message and they usually refuse to give you any feedback upon request
