I can only surmise why that is. Here are a few assumptions.
Germany has one of the densest road nets in the world.
It also one of the most used road systems. Thus repairs are necessary quite often, as are enlargements. So not all the construction sites you see are repairs, some are actually new construction or to make the road bigger.
The road quality is also very good, better than anywhere I've traveled except Switzerland. Much better than the USA, which I would largely call third world road conditions (the US civil engineer association gave an average degree of D-Minus to the road system, so that should tell you something).
It obviously takes much longer to completely dig up a highway, get rid of the rubble, re-level everything, put proper drainage and then rebuild it with a roadbed that is two to three times as deep as the typical roadbed used in the USA.
The second thing is that the state pays the money and the state is consistently short in change. There are no toll roads and Maut is taken only for trucks. Switzerland, Austria, Italy, England and France all have Maut or toll roads. In France they actually have private companies running the nation's highways. So when there is no more money, work may need to be halted.
The third point is that in order to build a new road there is A LOT of red tape and a lot of resistance from people who live along the proposed road. So it is possible that they start on a project and everything looks like it works out legally and all. But then some farmer gets a particularly inventive lawyer and they cannot continue building.
These are just reasonable assumptions as to why, what is not disputed is the fact that it sometimes feels like the entire country is a construction site.
Till