Don't conflate ratings and quality. The Fire HD 8 having a 4.6/5 and the iPad Mini having a 4.8/5 doesn't mean they are about the same quality of device it means they met the expectations of the buyer about equally.
Many get the Fire tablets because they wanted a cheap way to access Kindle on a color screen. Even more love it because there are dirt cheap options to hand their kids. At 1/6 the price of a base iPad Mini it can even arrive DoA and be rated well since Amazon will just RAM without asking questions. People aren't always looking for solid metal frames with high quality software, many are happy to find the cheap piece of plastic that lets them watch a movie in the car and will give good ratings to the product that let them do that for the price of eating out for a night.