Because our entire civilization is built on recipricoal alturism, which requires reputation so that in the event someone defects it carries negative consequences to discourage defection.
But regardless i do understand the appeal. Dating apps suffer from basically being a low-information market place. There are of course the malicious people, which everyone has an interest in removing from the app. However even ignoroing that its a bit of a lemons market (if you excuse how dehumanizing the metaphor is). Its very hard to tell if someone is a good date just from their profile, and people who are good dates end up in relationships and exit the market quickly while bad dates stay in the market for a much longer time. Allowing some sort of review system does solve that problem - its worked in other domains, like uber drivers or what resturant to go to. So i certainly understand the appeal of why people would want this.