Because 1) having any reviews at all drives purchases because of a social proof effect (see Influence - Cialdini). 2) Amazon are not liable because of Section 2

Because 1) having any reviews at all drives purchases because of a social proof effect (see Influence - Cialdini). 2) Amazon are not liable because of Section 230 of the Communications Decency Act (platform operators shall not be treated like the content publishers who create information on those platform).