Because every set of circumstances is unique. The candidate, the research, the supervisors, the results, the thesis, the examiners, the day, and so on…
There are common aspects. Regulations, academic culture and people’s experiences suggest probable outcomes and eventualities.
Check the regulations, explore the way vivas happen, listen to your friend’s story – but don’t expect their story to be your story.
And don’t expect your story to be completely unlike any other story you might hear.