LogFAQs > #968261200

It's because of the laws of Copyright, and Trademark in Japan compared to America (and the west):

In America:
"To qualify for trademark protection, a mark must be distinctive, i.e. it cant just be generic or descriptive. Brand names and designs, for example, can be protected by trademark. Generally film titles only describe movies instead of distinguish or identify their source; therefore, they usually dont qualify for trademark registration. However, when a film or a television project is part of a series, the title may qualify. For example film titles such as Star Wars, Kung Fu Panda, Toy Story, or Endless Summer qualify for trademark protection even though the titles may just describe what the movie is about. Now the title is more like a brand. This is the exception rather than the norm with trademarking film titles or television project titles."
https://www.nadiadavari.com/can-film-or-television-project-titles-be-protected-do-copyright-and-trademark-law-protect-film- and-television-show-titles-if-not-how-does-the-entertainment-industry-solve-the-quandary/

Whereas in Japan:
At times, multiple movies or novels have the same title. For example, there are multiple novels written on the life of a woman. Therefore, even if the title is the same as a novel published by another company, it may be used as long as the content of that publication (book) is indicated. However, the title of a movie or a novel cannot be exploited on goods or service without authorization if the title is already registered as a trademark of another goods or service.
https://www.jpo.go.jp/e/news/kokusai/developing/training/textbook/document/index/Copyright_Law.pdf

Since almost all of these LNs become anime, and merchandise the title of the LN cannot be used again.
However to get the original copyright, the title needs to be unique enough, so if the title is not just a single word, then getting the copyright becomes easier.