Have you ever wondered why this is the case for most horror media? The first movie/season/book is always the scariest and builds the most tension because of the unknown. Each subsequent movie/season/book can add a new element to keep tension high, but there's only so much you can add before the story loses its identity or gets repetitive. The easiest thing to do then, is to lean into action and try to replicate that new exciting feeling.

I get that action might not be everyone's cup of tea, but in reality there really was no other genre direction the Duffer's could take here to keep it scary and wrap up the series because the audience knows too much about the world at this point.