Because there's a button for "Helpful" on each review, and the most helpful reviews more or less generally get voted to the top -- and there are almost always some really long, in-depth ones.
I don't even pay attention to star ratings anymore. I just read the top 10 reviews and get a pretty good sense of what specifically is good and bad about the product.
Even without fake reviews, star proportions were never that useful in the first place because they're not a random sampling of users, and people are way more likely to leave a review to complain than to praise, so it's biased no matter what.