You can do that by restricting voting to established users with nonzero karma, which HN already does. If a bunch of people who are providing the high-quality submissions and comments say a particular submission is high-quality, what are the grounds for doubting them?

It is not difficult to farm accounts for voting rings, and there is a significant financial incentive to do so. It's true that there may be some false positives, but without these protections the floodgates would be opened to startup marketing goons.