Computers make it easy to track such things. eBay's success -- enabling strangers throughout the world to trade with confidence -- was built on it.

Sure, it worked and then it stopped working. Upvotes, reviews, social media, and word-of-mouth have been co-opted by advertising and marketing.

One failure mode is when someone makes 1000 accounts and upvotes everything that benefits them and downvotes everything that doesn't. And you have to balance letting new users into the system with not letting the same user into the system twice, and with not requiring a picture of each user's passport. Or some orange guy convinced 1000 people that what's good for him is objectively good and what's bad for him is objectively bad and facts aren't real and words don't have meanings, which has the same effect as a user with 1000 accounts, but even scanning passports won't help you.