Manga/anime is art, but like it's been said before, it's overused. One of the reasons why, (to the annoyance of most more skilled artists) is that it simplifies things and is quite easy to produce once you get the formula down. I have to admit, the manga style was one of my first drawing styles, but eventually, I sprung off of it and moved into realism painting.
When you take into account how much work goes into realism or another style, and compare it to manga, manga is almost effortless; the bishie styles are almost a surefire way to succeed, even in the hands of someone with a small amount of skill.
Really, I think some artists get really put off by the attention the manga style gets compared to the relative ease of making it. Also, some of the groups manga attracts are less than desirable for some artists, mainly the immature, obsessive "otaku" teens, or the "weeaboos." (urban dictionary definition: A negative term directed to anyone overly obsessed with Japanese culture to the point where they become annoying. Most weeaboos try to impress others with their otaku knowledge. Another trait of a weaboo is their desire to "be Japanese". )