I think there are varying reasons for this, but I'll cover the main points I feel influence this the most:
a) Anime for adults is a niche medium no matter how we look at it. The fact that Japan has an industry which actually bothers to regularly produce animations for older audiences does not mean that it is accepted as a mainstream thing, nor does it mean it is particularly successful. The only difference between how animation is view in Japan and in America is that Japan actually has sponsors and production companies who are willing to try making more than just animation for children. The fact still remains that if you have a successful adult franchise from another medium and you want to adapt it into a visual form, doing live action still makes the most sense.
b) The anime industry is largely driven by big production companies who have their tentacles in various industries - music, manga, light novels, etc. It is not surprising that all of these companies target a younger and more juvenile audience. It is much easier to indoctrinate interest in more impressionable school children, rather than working adults. At the same time it also gives you more flexibility in promotion through word of mouth via social networking. So it is unsurprising that these companies would have their own self-interest as a priority and want to use anime to expand the reach of their own works in other mediums - which general target a younger demographic.
c) Japan is a much, much smaller country in comparison with America and China. This means that both the creative output and reach of such creative works are more limited. To add to this point, Japan is a country which has their own language, generally understood only by the Japanese people and those who choose to learn the language from the rest of the world. In comparison, when an English novel is written, it instantly has huge reach worldwide, before translations are even taken into account. When Chinese novels are written, there are multiple countries in Asia able to understand them, including China - the biggest country in the world. When there is a much bigger market, there are many more works produced. Especially when such works can be written by people in various different countries who share a common language, and be read by many people in various other countries as well.