It is entrenched in the (political) culture of Germany. As I said, metrics are difficult to compare, so determining the degree of corruption is always a poor approximation. Transparency International usually has some good data, but they are completely off when it comes to Germany.
We have cases were public officials got bribes and weren't convicted or even didn't lose their position. Our former, former chancellor Kohl still refuses to testify against people that were involved in illegal party donations for his own party. He promised to keep it secret. The public is just told that these dirty money trails are "political reality".
But these are just the higher profile cases. It is prevalent in the industry too. That can have advantages as business do cooperate very closely with each other, but corruption is the inevitable result. A start up culture like in the US would just barely be possible, because established players will get their hands on the money that is available for investments.
Additionally, EU initiatives against corruption that mostly tried to introduce transparency about financial transactions are always rejected by Germany, with dire consequences for other countries.
Officially there is no legal term for corruption at all. Using your position of power to further personal gains is completely legal if no other laws are violated. There was "Amtsmissbrauch" at some point, but that was scrapped by the Nazis. Contemporary politicians probably think that not everything they did has been bad...
Aside from that there is also a lot of child abuse and getting dissidents declared to be insane by state authorities, but that is another story.
The CPI is a corruption -perception- index and Germany can be quite discrete about corruption and there is a tendency of people to believe the state. Neither Hitler nor the GDR-regime could change that apparently.