I'd say it's because people generally trust someone with nothing to gain over someone with something to sell.
You expect a company advertising to present the product in the best possible light, they're trying to sell it after all. On Reddit, people expect to hear the good, the bad, and the annoying parts all at once. Even if individual opinions are biased, a whole thread of independent opinions often feels more authentic than polished marketing.
That's not to say manipulation doesn't happen on there either. It's just that people tend to trust a crowd of users arguing with each other more than a brand going over talkingpoints with itself.