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.
Think we need more specific examples of what you’re watching, but I don’t think it’s just “American tv“—watched plenty of anime that is guilty of what you’re describing.
Plot creep is real, just look at any webcomic that’s been going for more than 3 years. Looking at you (lovingly, c… years ago?) Questionable Content.
Depending on the anime, but they usually have Arcs, which would be a named Show on its own. Then the second arc is the sequel. But usually the characters are pretty different at the end powerwise. I guess the equivalent is a character growth in a drama and some reversion to their original unimproved selves are common
Are you avoiding the question on purpose? For all we know you just watch bad tv