If not showing repeat vacuums does not improve clicks, I'd be very curious if it reduces clicks either.

It seems like what you posit is only a mystery if we take it on faith that targeting works. Once that faith is lost, you can also approach it from the angle that if showing them something we think they want is just as effective as something we know they don't want, you're actually proving that "what we think they want" isn't working either.

I mean, given "we know you've bought a vacuum cleaner" and "we think you'd like a kettle" - if ads for either are equally effective, either there's a lot more people collecting vacuums than either of us would have expected - or we're entirely wrong about the kettle. And it seems to me that our blind faith in targeted advertising leads us to wonder why people want so many vacuums.