Why is doctoral thesis plagiarism sometimes forgiven by academia? I was recently looking at the rates of PhD revocations of people who were found to have plagiarized large portions of their PhD theses by Vroniplag, and it isn't that high.
Most people that were found by sites like Vroniplag whose PhDs weren't revoked are either established professors or are working outside of academia so it's kind of understandable that their PhDs weren't taken.
But there seem to be instances where someone plagiarized portions of their thesis and  they are still promoted by their Universities and given funding.
Why is PhD thesis plagiarism forgiven in some instances by academia?
I certainly won't and can't speak for all cases, but it is still true that a doctoral student is a student and is in a learning situation. Therefore, some universities will treat plagiarism as they do other sorts of misconduct and try to turn it into a learning situation so that the student can improve their practices and ethics. The alternative of failing the student and expelling them from the program is extreme, of course.
I would hope that when plagiarism is found in a dissertation, and agreed that it is plagiarism, that the dissertation gets corrected and proper citations and proper quotations made. But that is a hope, not a certainty.
Sadly, too many people get out into the world without really learning all that should be learned, and, from the prevalence of plagiarism questions here, I think this is one area that probably needs to be explicitly addressed somewhere in the educational process. But, too often, people just make assumptions and assume others make the same assumptions without reflection.