Politicians and other students caught in plagiarism? This is something I don't understand: how come many degrees have been revoked due to plagiarism after approval if Turnitin and many other software against plagiarism exist?

For example, there was a case in my country where a politician got his degree revoked when it was found that he copied from other student's thesis, the thesis of that student was in her college repository, how come anti plagiarism software didn't detect the fraud before approval?
Actually, it is not just politicians. The media only report on politicians, though. I work with the VroniPlag Wiki academic group, there are over 200 published documentations of plagiarism, and many of these degrees have been revoked: http://de.vroniplag.wikia.com/wiki/%C3%9Cbersicht (sorry, in German, but you get the gist). 

As others have noted, Turnitin and other systems are just a tool, not a litmus test for plagiarism. They can only find what their algorithms can find in the database they have been able to populate. There are very many false positives and even more false negatives. The reports are very difficult to interpret, to boot.

For your particular question, I know from my work testing plagiarism detection systems that very many repositories of dissertations are not indexed by the software. One reason is that they are very difficult to traverse (and each has a different structure). There only tend to be the dissertations stored in the databases that have previously been checked. So in your case, if the student's thesis A was not checked, it was not in the database when B was potentially checked. But if a human reader sees that the topic is the same and then compare's thesis A with thesis B, they can quickly see and document the overlap. And of course, not all theses are checked, for various reasons, not all of which are academic in nature.

If you want to know more about the work that goes into documenting plagiarism in (oldish) theses, I blogged about that ages ago:  https://copy-shake-paste.blogspot.com/2012/10/how-to-find-plagiarism-in-dissertations.html (in English).