Hawkins Lab after the disappearance of Will Byers and Eleven’s escape.
Season 2 saw the aftermath of those events, as well as the gradual invasion of the new and improved Mind Flayer, but also proved that danger can take human form as well, and the gang came face to face with a group of Russians that established a secret lab underneath the new Starcourt Mall. Them, along with the Mind Flayer, created major chaos and more destruction than in previous seasons, with Starcourt Mall being burned down.
At the end of season 3, and thanks to a TV news network, viewers learned what happened with Mayor Larry Kline and the version given to the public about what happened at Starcourt Mall. Even though they obviously covered the truth and watered down the information, there was so much chaos that at least a couple of residents should have seen what truly happened, but that won’t be the case.
Stranger Things: Adults Must Be Oblivious To What Happens
In three seasons, Stranger Things has shown that the majority of Hawkins’ residents don’t care about what happens in town as long as it doesn’t affect them. Will’s disappearance was a widely known case, and there were search parties all over town. When his “body” was found, it was also all over the news, as was his “return from the death”, so they were all surely aware that someone put a fake body in the quarry to put an end to the case. In season 2, at least a couple of residents knew there was something strange going on as the ground and trees seemed to be rotting (all because the Upside Down was slowly taking over the human dimension), and Barb’s parents were told their daughter died due to a “chemical leak". Last but not least, the destruction of Starcourt Mall in season 3 caused by a giant monster who chased part of the group down from Hopper’s cabin all the way to the mall surely couldn’t have gone unnoticed.
All these moments have made fans wonder why people living in Hawkins don’t seem to care about what happens. Narratively, it’s a trope that is often used in movies and TV shows with children (and teenagers) as protagonists as it makes it possible for their adventures to happen – surely, no parent would ever let their kid fight against a giant monster (inspirations behind Stranger Things: Stephen King’s IT, where adults don’t pay attention to what their kids are going through and thus have no idea they are being terrorized by an evil shapeshifting entity.
As for an in-universe explanation, it could be simply because the adults in Hawkins don’t care about what happens around them as long as they are safe, which is not that far-fetched when looking at real-life society. A good example is Mike’s father, who pays no attention to his kids (nor his wife) and how they are doing, not even when the people from the lab arrived looking for Mike. With this in mind, and even if some residents saw the Mind Flayer destroying everything on its way, it’s unlikely they will start caring about what happens in town – especially because the news said it was something completely different. The adults in surprises in season 4.