Amazon does do a lot to detect bribed reviews automatically. However, in the long run, this may be an arms race that Amazon will lose.
One issue with detecting fakes with identical phrases is that many real reviewers will take phrases from the product description or from other peoples' reviews either consciously or unconsciously.
This also matches up with a common bribed review technique of giving people canned phrases to repeat in their reviews or hundreds of prewritten reviews to be distributed out to people who are part of the review program.
Top reviewers are also not a good brake on this process because the best way for them to profit from their reviews is to take bribes. Just going after people who only leave positive reviews also doesn't really work because smart reviewers salt their reviews with the occasional negative one (generally for people who aren't bribing them).
User generated reviews emerged because many surveys across many different product categories have shown that shoppers tend to trust peer reviews more than they trust expert reviews. Part of this is because so many expert reviews have been corrupted to an even greater degree, with PR departments basically being bribery and party-throwing departments whose jobs are to manipulate journalists and place articles in supposedly unbiased publications.
The issue from the seller side is that shoppers are absurd perfectionists. A product can go from a 4.24 star to a 4.26 star average and have its conversion rate double. The difference between 4 and 4.5 stars is the difference between losses on the product and profits in many competitive categories. Also many people just give 1 star reviews to products when they have minor nitpicks. That'd be fine if that 1 star review didn't tank a product average and suddenly mean the budget that was there for an employee suddenly isn't there anymore.
In real terms the main difference in most categories between the reliably 4.5-5 star product and the 4 star product is that one seller spams harder than the other one. Considering that nice guys finish last in that particular competition, it rapidly becomes a triumph or the worst.
Long-run what will happen is that the reaction against user reviews will be as severe as the reaction against critics has been. Part of this is also just a scale issue: with so many products and brands constantly popping up globally, there isn't enough time and editorial attention available to review everything.