It's more of a stepping stone. Generally in German coalition governments, the smaller of the two parties (things get more complicated when there are more than two parties) gets to pick the Vice Chancellor who is also the Foreign Minister. But Minister of Defense is normally more of a "shut up and don't bring any attention to yourself while you pad your resume" position than anything.

The German population is generally not too excited about its military (which was originally abolished after WW2 then revived as a defense force in the Cold War, expanded to include peacekeeping missions in the 90s and only post-9/11 again expanded to more direct involvements in ongoing military conflicts as in Afghanistan) so you can't really win any hearts and minds in that position.

There have also been numerous dodgy attempts at modernizing the military which broadly failed for various reasons (poor quality in purchased equipment, etc) and more recently a number of scandals with far-right extremists in the military. Basically any attempt to do something noteworthy in that position have historically been more likely to cause trouble. Ursula von der Leyen was actually fairly okay but her legacy was mostly in her work on accomodations for women in the military, which was seen as a continuation of her previous tenure as Family Minister, which was seen as too feminine for the small demographic which actually cared about her job.