Most of these backstabbers only came out of the bushes after the election. Since Trump only had a few more days left in office, and was occupied with dealing with the obvious massive fraud employed by the Democrats and other players both foreign and domestic, for Trump to 'fire' anybody at that point would have been seen as petty and vindictive. Regardless of how overwhelmingly justified it may have been, it would have been reported as Trump 'lashing out' because of the election. By the time things came to light, it was too late for Trump to do anything about it. Again, which was the whole point. That was the plan all along.