I am not sure if that was rhetorical, but reviewing things a) takes time and b) builds up Amazons moat with no benefit to the reviewer
It doesn't take much time, no more than a minute or two, I don't buy much. No its true I get no value from it, but there's many things humans do they get no direct value from. I read Amazon reviews, I get value from them some of the time, I do use them to inform my purchasing decisions - I'm paying it forward.