I have been an Amazon customer for 20 years but have never created a review, even though I often rely on the reviews which other customers have provided. Why not? Partly, I don't know how useful my opinion will be. Partly because to give a review, I feel I should have used the product for some number of months. (In your example, your review is based on the timely delivery of the books, and their condition as promised. Sure, this service could be reviewed immediately.)

Mostly I don't review because my time is limited, and these days I prefer to answer questions on an expat forum to help new arrivals in my country. And, I correct notebooks in Italki by residents of my new country. I also have personal projects to work on. It is a matter of which hobbies do I like!

In my opinion, all reviews are subjective, and some are objective as well. Strangers have all sorts of biases, motivations, and even attitudes.

I strive to be objective when judging reviews. I will discount reviewers who sound dumb and biased, and I will discount all reviews if a product just has a few. Only when there are dozens of reviews that I can average will I feel they they can be trusted. Then, back to the beginning, a reason I don't provide reviews is that mine will be a drop in the bucket of the dozens or hundreds that are averaged, and I would rather spend time writing to help one person directly.