Answer:
Celebs are idols. In society today a lot of girls and guys look up to them so when they wear clothes to any kind of event, then the clothes they're wearing seem "in" and cool so those kids go and buy clothes and other kids see that and think its cool and on and on and on. In reality, they celebs have stylists who put their clothes for them, so the celebs might not even have a fashion sense at all in real life.
Actually the real fashion trends happen on the runway. The most influential one is the Mercedes Benz Fashion Week in New York. It comes seasonally in the Spring/Summer and Fall/Winter seasons. so the stylists get the inspiration from the runway and just do it on the celebs.
that's why i strongly think that celebs being this so called fashion trendsetters is disgustingly overrated.