I don't know why you're being downvoted when it's clear your question is out of curiosity.
High effort reviews are common on Goodreads, why? Because it's a community about passionate readers and probably those who write the very detailed reviews have enjoyed reading some other reviews and want to contribute back to the community. I think it's the same reason as people contributes to wikipedia: for the sake of sharing knowledge
High effort reviews are common on Goodreads, why? Because it's a community about passionate readers and probably those who write the very detailed reviews have enjoyed reading some other reviews and want to contribute back to the community. I think it's the same reason as people contributes to wikipedia: for the sake of sharing knowledge