Turns out the more times you hear something, the more likely you are to believe it. Repetition is the key and it doesn't seem to matter much whether you hear it from multiple people or via multiple media or from one person multiple times. A study by Kimberlee Weaver and colleagues published in the Journal of Personality and Social Psychology found that one person in a group expressing the same opinion three times had 90% of the effect of three different people in a group expressing the same opinion. (Weaver et al., 2007).
So how does that play out in our daily lives? Advertisers and influencers repeat the same message over and over. Familiarity doesn't breed contempt, in fact it does exactly the opposite - it breeds attraction. When an opinion is repeatedly broadcast at us by the same organization, not only do we believe it, but we're also likely to believe it represents the general opinion. That's despite the fact it is analogous to the same person repeating themselves over and over again.
Where does this effect come from? The study's authors argue it comes down to memory. Because repetition increases the accessibility of an opinion, we assume it has a high prevalence. In everyday life we are likely to hear the same opinion many times in different places. We then put all these together to judge the general mood of a group. When one person repeats their opinion, we simply over apply the rule.