I think that is because a lot of the review technology is designed to specifically stop this kind of stuff from happening. If it were easy to do, then you could effectively flood a service/product with bad reviews to destroy its business, or spam out a product with good reviews even though it was shit. Amazon has a lot of time trying to stop that (well, they say they do) and it still happens.
So, people will need user account, prof of ID or a captcha or something to try and validate that the review relates to a person and an actual event that is legit.
Of course, AI will totally screw all this