>>281426625
For the simple reason that they don't have time to plan it, the Japanese entertainment industry is very competitive, and often you only get 1-2 opportunities to truly stand out, so they put all their effort into the beginning, a hook that attracts audiences, a novel, shocking, or controversial premise, no matter what it is, the point is to stand out. They'll figure out how to continue the story once they've signed a big contract.

It may also happen that the author had a planned ending, but due to its success, their editor demands they milk the cash cow, even if it ruins the entire story. For example, SnK was planned to end with all the cadets and Eren dead, no politics, foreign forces, mystical Titan powers, just a dark world with a depressing ending. Perhaps not the best ending, but it maintained the tone of the story, very similar to their one-shot. (You can look it up, there are interviews.) But people asked for more, and look at the result.