Because you prove the assertion, not the negative.
Where’s your evidence that there isn’t a flying spaghetti monster? You don’t have any, so there must be one!
Where’s your evidence that there isn’t a invisible pink unicorn? You don’t have any, so there must be one!
When you successfully and conclusively prove the non-existence of the FSM and the IPU, I’ll show you the irrefutable proof of the fakeness of the video that I have in my other hand as I type this.

