Your examiners are invited to be at your viva. Like most invitations they can decline if they need to or want to.
A viva is a much bigger commitment than the hours on the day for an examiner. If they accept and attend it’s because, more than anything, they find the invitation to be compelling. There is something about your work as it’s been presented that makes them think it will be a good use of their time.
Reflect on what your examiners might be seeing in your work. Focus on that as part of your preparations. How can you share your research with them? What do you need to do to be prepared? And how can you make sure that the invitation they’ve said yes to results in a good experience for them?
When you have ideas for that question you’ll also have ideas for what will help the viva to be a good experience for you too.