I haven't really noticed fewer book reviews. However, like you, I am always stunned at the low number of reviews for many books that are excellent and deserve to have reviews.
I don't know if you've seen this, too, but in a course I took about selling on Amazon and elsewhere online, I've heard that only a very small percentage of buyers actually review books or products, sometimes as low as only 1 to 2 percent of buyers. I'm not sure if these low numbers are due to the review process being inconvenient or people are too busy.
I've made a commitment to review every book I read to support my fellow authors. Even if readers only did it for half the books they read, that would be great.