In addition to what Arcane said in their answer, I think there's another major factor:
A lot of anime shows are based on manga, which themselves may or may not be built for a long-running series. Some of the things published as manga are simply not created to be an open-world for exploring stories.
For example, consider The Green Mile written by Stephen King (I know, not a manga but go with me here). It is a wonderful novel which was originally published as a series. I mention this because it was a serial novel, but it wasn't a long-running serial novel—which is similar to many manga. Each piece in the series plays its part, but it decidedly reaches an end, and that's that. There's no room to expand on the story of The Green Mile. The author told the story he wanted to, and there's nothing left to talk about. If they continued to produce 'episodes' of the series that talk about other prisoners, or other events in the same prison, I feel it would sort of lessen the story told in the 'original'.
(Side note: if you like the movie for The Green Mile, but haven't read the book, then I can't possibly recommend the book enough. It is by far one of my favorite books/movies of all time.)
Some mangas are like that—they have a story to tell, they tell it, and then it's done. There's no more. Things like Death Note come to mind in that regard. The manga series has ended, and the story has been told. I don't know the chronology of the releases for the movies, TV episodes, and manga, but it's all 'done' at this point in time. It's a good read and a good story, but I think it would be lessened if they had tried to crank out episodes that fit the universe, just for the sake of making episodes.
Some shows are great for regular, periodic episodes. Others just tell a story, and when the story ends there's nothing left to do but move on. In fact, I kind of hate it when they 'test the waters', like Arcane mentions, and season 1 ends on a cliffhanger because they were seeing if season 2 would be a thing. Then you get a story that's left unfinished, and you may not get the end at all (from the show or written material). If it's one thing I hate about a story, it's an unfinished story.
Just imagine if a show like Breaking Bad had just continued until viewership fell below a certain threshold, and then the next season doesn't come out just because it wasn't approved by the network. That would be horrible, and the ending that Breaking Bad had was some of the best TV I had seen in recent years. Sometimes getting that complete resolution to everything is the best thing for the show, series, and audience.
So, to bring this all back to your question: I think part of what you're seeing is a lot more authors trying to tell a specific story, and doing it well enough to become really popular, but then the story is done. There is a lot of manga out there these days, so the market is saturated with really great stuff to read/watch. Getting a long-running series going then hits the issues that Arcane mentions, so what we end up seeing as 'successful' end up being the short/limited run series which may only run for 13-25 episodes, but they reach their conclusion and the audience is happy. And we consider an attempt at being long-running to be a 'failure' when it doesn't make it past the first or second season even though the episode count is the same as the short/limited series.
And there are only so many hours in a day, so at some point that comes into conflict with how much people can watch. If we all end up watching really high quality anime that is 1-2 seasons long, then we simply don't have time for that 5+ season series. Perhaps there are such series out there that just haven't reached your attention yet, or maybe if you did see such a series you would think it's 'too childish' and let it pass you by.
Small note about Attack on Titan: there are still more episodes coming, but it seems to have a long cycle to create, much like Rick and Morty. Perhaps continued support and revenue will allow them to hire more people to speed things up, but that's an issue with their business/development cycle more than it is with the content. You could have a super awesome business idea that would make you billions, and let your descendants live in luxury for generations to come, but it doesn't mean squat if you can't get it to market. Unfortunately, there are likely many great untold stories that fall into that category for one reason or another.