Who is responsible for detecting plagiarism in a PhD thesis?
The problem is that the PhD system is designed for people who intend to become researchers.  For these cases, plagiarism is not at all a common problem.  You are expected to published your research, and you will not have a successful career unless it is widely read and cited.  That gives lots of opportunities to get caught, and the penalties for plagiarism are a huge deterrent.

To the extent you find plagiarism, it's generally people who do not want a research career, but instead view the PhD simply as an obstacle on the road to a teaching (or other) career.  Probably the community should scrutinize these sorts of theses more carefully, but it can be hard to work up the energy to do so when most of them are OK, and when these theses really don't matter much for the research world.

The German politicians are pretty much the worst case scenario.  In the US, the stereotypical case is educational administrators.  Typically, you have a distinguished person who starts to feel the need for a PhD.  Perhaps it's because they associate with academics and feel looked down upon, or perhaps it's because an academic endorsement would make the public value their expertise more.  This student is very smart and accomplished, and nobody suspects them of any dishonesty.  However, they are also very busy, often working on a PhD while pursuing other projects as well, and academic research is not a priority.  At some point, they succumb to pressure and start taking shortcuts.  Probably it starts with small things, but the shortcuts gradually grow larger.  They rationalize that the thesis doesn't really matter anyway, because they have no intention of following an academic career track.  After all, they have the knowledge and experience, and they deserve the PhD title, so what difference does one document make anyway?  Meanwhile, the advisor probably doesn't spend that much time working with the student, and has no reason to suspect anything.  The advisor really ought to be extra careful in cases like this, but that would seem like an insult to the student, so it's easiest just to trust them.

So my take on this is that plagiarism is not as widespread as news stories might suggest.  It's just particularly likely to happen in cases where it would attract media attention.