They're not trying to destroy your thesis, they're trying to make sure it's complete and sccurate. In theory, the only person who knows the material is you, so they can't just ask anyone else.
I never really understood why they call it a defense. Nobody's going in there trying to insult or fight it.
How science often works is you try to disprove things, and if you can't, you accept them as likely to be true. So, to show that the thesis is complete and accurate, they're trying to find places where it's incomplete or inaccurate. In the defense, your job is to defend against these attempts.
Scientific works should ideally stand on their own and present all the required information to understand its conclusions in a easy to comprehend for form.
Requiring a human, who is more prone to emotional and subjective errors during live speech then during honest predetermined writing sessions is everything but ideal.
However, a scientific work should be the most possible comprehendible to the target audience which may require multiple mixed forms including live discussion of subject matter.
[scientific medium that allows people to learn and grow independently of other factors] > [scientific medium that aids learning and growth in combination with other factors] > [scientific medium that requires near full knowledge of the subject matter to understand (thats just art for smart people)]