Because the US government isn't captured by corporate interests? They rank significantly worse than Germany on the Global Corruption Index [1] and on the Corruption Perception Index [2]. In a global comparison both are great, but relative to each other the US really isn't an example Germany can look up to.
The chance of a government staying in government for a long time also means that governments have to think long-term because they might still be in government when the consequences come around. In the US system the optimal strategy is to do things that look good in the short term but backfire as soon as your term limit expires. That way you look good, and the next president (who's with near certainty from the opposition party) looks bad.
I'd be more in favor of passing an age limit, but I'm well aware that that has no chance of happening.