There are a few reasons for this. First, many companies use Amazon as a way to market their products. They may provide products to Amazon reviewers in exchange for a review, or they may offer incentives to reviewers who write positive reviews.
Second, Amazon itself has an interest in promoting products that are popular with customers. This means that products with a lot of positive reviews are more likely to be promoted by Amazon, even if those reviews are fake.
Finally, it's easy to fake reviews on Amazon. There are a number of ways to do this, including creating fake accounts or paying people to write positive reviews. This makes it difficult for customers to know which reviews are real and which ones are fake.