Yeah this is so frustrating. Happened to Stranger Things, The Fall, Homeland, 24, Criminal Minds and so many others. I often think it's because they start off with a good idea, interesting characters etc. and then after a season or two the main plot lines or character development has been done.
But by that stage the audience is invested and it gets good viewer numbers, so the studio throw a load of money at them to keep going for another few seasons and it's just a rehash of the same shit. Or they get the characters together, which never really works.
US shows are really bad for having loads of seasons and so many episodes, there's only so many stories you can do. Or if it is built on a certain premise then only takes a season to unfold, then they take it in a weird direction that doesn't work and flog the crap out of it. It's all about money I guess. They want to cash in on a sure thing.
It's the classic business vs creative tug of war, you see it in movie franchises and music as well - something new is successful because it's fresh and different, then the guys in suits want 10 more of the same thing because it was a hit, and they lose what made it interesting or unique in the first place.