It is clear that many people were involved in conspiracies, and that their negligence directly lead to many deaths, as they knew that it would.
Why hasn’t the current administration brought criminal charges against them?
Also, the courts can force non-government employees to testify, and most of the evidence in the report seems to have come from non-government employees. Firing them doesn’t somehow absolve them of their crimes.
No idea. My guess is that it takes time, and that the DoJ will need to be prodded to do it, that the DoJ won't do it of its own volition. Whether the AG or deputy AG have to get involved, or the POTUS, or perhaps a criminal complaint has to be filed by, e.g., Ethiopia, I don't know, but I expect there are no DAs who would initiate a case here on their own (whether because government protects government, or because they think the case is radioactive, or whatever).
Sure, if the President had fired them earlier that would have been satisfying -- at first, then when they all lawyered up and refused to testify, we'd all have thought that the President was a moron for having fired them too soon.