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.
I suppose it’s the natural result of wanting to keep the show on as long as possible, when you’ve only got one good idea for the story arc. You need a lot of filler.
I’d like to see more shows done in the style of Babylon 5, where the creator had the whole 5 years written out from day 1. There was very little in the show that felt like filler or treading water.
Which also may explain why books are being brought to TV more frequently these days. But, TV showrunners have a bad habit of taking a good novel and totally mangling it in the translation to TV, so it’s not a guaranteed win.