This is an excellent (and common) question. The key thing to remember here is that it is the platforms themselves that make the determination on whether something is removed. The very purpose of those platforms is to provide real, informative reviews for others to read – so their preference is very biased towards keeping reviews up rather than taking them down. They therefore only remove a review for a violation of their policies – and even convincing them to do that is quite a lot of work.
The fear that a genuine bad review might be taken down is understandable, but people with this fear should also consider exactly what makes a review truly legitimate. The author might be real, and they may well have actually been a customer of the business; but their review still must follow the content policies of the platform. If it is abusive, harassing, lacks any real information, or any other policy violation – then it is still a candidate for removal. As much as people are used to being able to say just about whatever they want on the internet – the review platforms are well within their rights to request that the people who wish to use their totally free services follow some rules that aim to provide a quality experience for everyone.