I think the difference is that celebrities and "influencers" are real (-ish) people, so if they say they recommend something, that's an actual piece of anecdota

I think the difference is that celebrities and "influencers" are real (-ish) people, so if they say they recommend something, that's an actual piece of anecdotal data when deciding whether or not you want to buy into it yourself. If somebody in a movie says they're going to Fyre Festival, that might loosely associate the thing in your mind with "coolness" or whatever, but you're not going to mistake it for an actual endorsement from a real person.