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.
Showrunnners are never absolutely sure how many more seasons they’ll get. If a show is popular, they could end up having to continue it after a conclusion. Or the show could be popular but corporate priority could be elsewhere, and they’ll be forced to wrap up promising storylines quickly. Even for shows that announce they have plans for a beginning, middle and end, it’s possible that they’ll be cancelled before end planned ending, or else have to stretch after the ending has been reached. Safer is to try to just coast along, being non-committal about major plot elements, until something happens that pushes the show to resolve things.