Season 1s are great, setup, some payoff, a bit of lead into the overarching story. Then season 2 to X. The heroes win and then lose in the final episode, cliffhanger to next season. People get bored. Final season is announced and they wrap up the show.
Really great shows have a broader plot premise and are free to build new storylines and character arcs each season. As YoBuckStopsHere said, some great shows build up and grow overtime - think Breaking Bad, Parks and Rec. Both shows start off slower, focus on character building in the earlier seasons. Then they become plot focused later on.
Other shows have the flexibility to create new story arcs so each season almost stands on its own but they still stay within the larger overarching premise, example - The Great, Game of thrones (although they really gave up at the end)
I think good shows have a plan for how to get to the end and mediocre shows do as OP described - have a beginning and end planned and not much in the middle. I don’t think all shows are stuck in Act 2, but it does say something that the ones that aren’t stuck there stand out that much more