Another problem is the healthcare system and the chances of one to actually afford it when the time comes - HN had a few good articles about the subject.
And there may be other somewhat lesser problems too.
I'm much more worried about things like social & political stability, cost of living, and if I'm being perfectly honest, weather & food.
Germany has very good schools. By denying your child the ability to attend, you are committing an offense to the child. Why should this be allowed?
Also, a shorter answer to your question: Because Germany signed the UN Declaration of Human Rights. See Article 26(3).
The point is, it should NEVER be illegal to refuse to send your children to a militarized system which has stunted and de-humanized every single person in the modern system. In the UK, they ever tried to make Summerhill school a bunch of criminals. Also see Steiner schools, unschooling etc etc. The evidence would indicate that doing the BEST thing for your children would make you a criminal in most first world states. If you wonder why the electorate has become more malleable, "stupid" and gullible in the last 100 years, maybe you should look into why this is. It's not because we are "devolving" or because people are stupider now. They are forced, trained and educated to be thus.