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.
It can certainly seem that way sometimes. Shows like The Handmaid’s Tale have been circling the drain of their own premise for a few years now. A big part of it, I think, is that they want to keep their main cast for as long as possible, which limits the options of what can happen.
Give me a mini-series, or even an anthology series, any day.