How long ago was it the last time someone like Gandhi incarnated as an Anunnaki in an attempt to save them? What happened that such interventions have not happened since?

We can tell you that there have been and continue to be other beings, including humans, who have incarnated as Anunnaki and other extraterrestrial beings as well to be change agents, to be warriors for the light working from within their own ranks to be an influence on things, to be a source of new ideas and perspectives, and create a possibility of a shift to a less depraved perspective. These individuals have all had an impact on things to a greater or lesser degree, but it is slow going. It is difficult to gain much traction when the whole of a society is geared towards serving a ruler, a supreme leader, who holds most of the power through heavy manipulation and control of all in that society. It is a much more difficult challenge to inspire change in such a society than it is within most human cultures, but yet there are high-level beings who do this service work with great joy because of the potential gains and their importance for not only the Milky Way Galaxy, but the universe as a whole as all will benefit when the problem of evil has been solved.