> That is, there's some friction for the malicious actors.

Fake reviews are just another form of marketing, like traditional junk mail. Making mail spammers pay postage doesn't seem to have changed the fact that most of the snail mail I get is junk, even in the digital age. Charge reviewers a quarter or a dollar to leave a review. If I was passionate enough -- one way or the other -- to leave a review, I'd pay it! I'll bet it would squelch the fake reviews overnight. Just tune the charge to find an acceptable signal-to-noise ratio.