Manga should not be considered a Japanese comic because it has several genres and a wide range. Also, its plot is different than typical American comics, so the term comic is misleading.
A manga is not a comic since it mainly it has more genres, such as romance, supernatural, fantasy, and many more you’ll usually find in a book or TV series/movie. It also created its own genres which have been adopted into book and TV genres. The first one is LGBT parenting, which involves parents of the same gender taking care of a child, second yuri which means girl x girl, and the last being yaoi boy x boy. Yaoi has become more popular of the three because it is mostly for the female audience, those really into is are given the title fujoshi.
Another reason is that its main target audience is teens and adults, not children. There are a lot of graphic scenes in manga that a child would not enjoy nor understand but a teen or adult would be able to get the main focus of it. The three genres created are meant to target the older audience since it brings a little humor along with it.
The final reason why manga should not be considered a comic is because of plot. In an American comic each chapter/volume has its own plot while in manga the series has one plot, and oftentimes to continue the story but with a new plot they change the title of the whole series and continue the story.