Book reviews are very important for authors, especially those of us who are not household names. Not only do they help potential readers decide if a book might be to their taste, they also have an effect on the algorithms. The more reviews, the more a site promotes it.
One of the most common reasons readers don't leave reviews is because they have the mistaken idea that they are expected to write it as though they are a professional literary critic. Nothing could be further from the truth.
To write a book review, the reader only needs to keep three questions in mind:
1. Did you like the book?
2. What was your favorite part? (no spoilers!)
3. Would you recommend this book to others and why?
Putting the answers to these questions into a coherent paragraph is all an amateur reviewer needs to do. Failing that, at least leave a star rating. Even a review that consists only of "I really liked this book" is better than no review at all.
Another reason some might not leave reviews is because they are intimidated by the technology and don't know how to do it. It's actually very easy. Here is a Youtube video explaining step-by-step how to leave a review on Amazon. The steps for leaving reviews on other sites are similar.
Happy reading and reviewing!