Because they are engineered in Germany, by Germans.
German engineers have a habit of building stuff that only just works. Their designs tend to be overly dependent on high precision of critical components and as a result these components must be, and are, manufactured to exacting tolerances.
The problem is they also tend to use average quality materials, or even materials not suited to the application, which are subject to environmental factors and premature wear. Once a small amount of variance occurs due to wear and/or environment, the tolerances go out of specification and the item no longer works as designed, or fails completely.
The Japanese on the other hand give careful consideration to the effects of wear, and design with the intention that normal wear and tear will not adversely affect the component’s operation. The engineering often appears to be of a lower standard than the German part, but in reality it doesn’t require such high precision due to the more wear tolerant design.
I have been involved in the repair of many types of equipment from many countries, from cars to power tools to electronics, and the German designed components almost invariably suffer from the same types of issues as detailed above.
A prime example is this Bosch demolition hammer:
The engineering inside these things is beautiful, but they are incredibly intolerant of wear. The internal components only need barely detectable wear for the machine to have lost significant hammer energy. They are very expensive to repair and don’t last nearly as long as the precision machined components suggest they should.
Then there’s these:
Hitachi build these things to go a few more than the extra mile. I have seen these so badly worn internally that I wonder how they could possibly be working at all, but the owner's complaint is not about loss of performance. They’ve come in for repair because they’re getting a bit too noisy, or have developed a minor electrical problem.
Of course there are always exceptions, and sometimes the Germans will deliver a product that is outstanding in every respect. They just don’t do it nearly as often as the Japanese, and it seems to be that they just simply don’t give enough consideration to the normal operating conditions that their products will encounter outside of their R&D labs.