[mikebenfield]

> Failing a PhD thesis during the defence is apparently a 'rare occurrence' in most universities.

Yes, but not for the reasons you think. Typically the advisor and thesis committee simply will not let the student defend (or at least will strongly urge the student not to defend) unless they are extremely confident the student is ready and will pass. A student failing the defense would be embarrassing for everyone, and so the real judging happens well before that. These days the defense is mostly a formality.

Moreover, students are not even admitted to the program unless they show very strong promise of being able to complete it. If you want to look at low success rates, here is one place to see it. Very few students graduating with a BS in any given field (computer science, mathematics, whatever) have any chance whatsoever of getting admitted to a decent PhD program in that field.

Why is it so hard for you to believe that getting a PhD (something you've never done and know nothing about) is hard?