That's kind of the idea. Sellers are gaming the system. They want buyers to assume that they got that fifth star because their service was indeed excellent (when it wasn't). People buy into that, subconsciously or not. And four is better than five, so if you see a seller with five stars you're likely going to choose them even if it doesn't mean a thing more than sellers with four stars.
Because people assume.
If everyone (well, almost everyone) is ranked at (or, expects to be ranked at) five stars, how can you tell the really good sellers from the mediocre ones?
Geographically, and culturally, I'm European, (and, moreover, I used to be a teacher, which meant a lot of experience grading student essays and term papers), and thus, I have no difficulty in not awarding the highest mark, or grade, to everyone, especially if they do not merit it, on the grounds of goods or services.
My internal mental marking system is that one awards three stars when everything is (or has been) perfectly fine, the service was as expected and ordered, and paid for, and the goods (or service) was/were delivered without any problems, and thus, there are no complaints; it is perfectly adequate, there are no complaints, but there is nothing extra or worth noting, nothing special to this exchange. A classic C, in other words.
Fours stars is good, a grade that signals that this is better than average, or, that the product is particularly good, or has been delivered promptly, efficiently and/or with courtesy; what one would have graded, or classed, or ranked, as a B.
Five stars means something well beyond the standard norm, the delivery of exceedingly good products or service, something almost exceptional - in quality or standards. In other words, an A.
Now, I cannot conceive of a universe where every exchange of goods and servcies gets to be ranked - and worse, expects to be ranked - as excellent and exceptional, for that is not how things work, especially in the profit driven world of commercial transactions.