Biggest reason is simply quantity, Japan churns out hundreds of animated stories from new authors every year, the west do barely anything in comparison.

When you create that much content some of them do become hits, and it also encourages more authors to create more content, while western studios only invests in franchises they can control themselves to create another marvel, while Japan just churns out content while the authors retain the rights.

Edit: Many western authors writes about other worlds etc, they just don't get anything animated since it is so hard to get anyone to invest in your story.