For the same reason I would show someone a “totally real and honest” screenshot of my company’s finances for some quick cash. Doubly true if there’s effectively no way to be caught.
Is your claim that these studies are incapable of measuring what advertisers actually care about? That Google is reporting fraudulently positive results? Something else?
(Once I understand your claim I'll be better able to respond: I think "there's effectively no way to be caught" is not true under both of the interpretations I see)
Offline purchases are harder, of course, but pretty sure you can still do this: https://support.google.com/google-ads/answer/9994849