require the students to defend their work in a viva voce and a written final.

Time & effort, primarily.

Also equality is a problem. Academics are human too, and tend to have preferences regarding looks, sex, nationalities etc that are hard to ignore. I had female students rub my legs in presentations, wear super short mini-shirts, see-thru top with massive clevage etc. Preventing this is a major advantage of anonymous grading. These days you want to protect yourself against harassment lawsuits. I feel very uncomfortable with non-anonymous examiations even though I agree that from a pedagogic point-of-view they preferable, not just to deal with cheating, but also because they give a much better view of the students strengths and weaknesses, leading to much better feedback.

If your students are behaving like that then run of the mill cheating is the least of your problems. And I can't see how a thesis can be properly defended except in real time, we aren't talking about marking an exam here but getting the student to make it clear that he or she really does understand the work presented (in my case over a hundred pages of experimental design, execution, results, and interpretation) and can explain it and defend it against criticism.

Some universities still require PhD theses to be defended in public, for instance Oslo: ==== begin quote

The PhD degree is conferred on the basis of:

completion of the programme's educational component

the doctoral thesis

the doctoral examination

The doctoral examination consists of the trial lecture and the public defence of the thesis, also known as the disputation.

=== end quote