At the same time, building weapon systems employs your people and sends money to your companies. So, this is not like Germany is giving hundreds of billions away and getting nothing. In reality it is sending federal money right into its own economy.
Injecting hundreds of billions into the defense industry might create only a few thousand additional jobs, whereas the same investment in capital expenditure and competitiveness could generate hundreds of thousands of jobs for an export-driven economy.
Russia, in its current state, is no match for the combined strength of the EU, let alone the entire NATO alliance. To even pose a threat to Germany, Russia would first have to get through Poland, which is rapidly expanding its military and investing a significant share of its GDP in defense. After three years, Russia has still been unable to break Ukrainian resilience, which makes the notion of it pushing through Poland with ease and threatening Germany even less realistic.
If I were German, I wouldn’t want to see my hard-earned money tied up for decades in barracks and equipment that will likely gather dust, given there is no plausible scenario in which Russia could directly endanger Germany.