Perhaps the artists who think that reviews are so important, could, indeed, ask for a review in the model description.
"If you like my work, please leave a review. This will help me to keep on designing more models, like the one you downloaded."
It's asking without bugging the customer with an email or personal message. If the customer doesn't read the model description, then they're probably not the kind of persons who would consider writing a review anyways.
I think the main reason people don't leave reviews, is because they are bombarded with review requests with everything they purchase. For example, I always buy my PLA filament and photopolymer resin on Amazon. I write a review on them the after the first time I order them and found them to be a great product, but Amazon constantly bugs me to write a review every single time I order the same product. Another example, is that I buy some replacement controllers for my son's Wii. What can I write in the review, except that "They work". Shouldn't that be an expectation?
If I'm planning on buying an expensive model, you bet I'm going to read the description and reviews to ensure that it suits my needs, but for something $10 and under? Forget it. It's not worth my time, unless it's the "be all-end all" or "life-changing" model. LOL.