He wants to. He wants to fire everyone who is competent and tells the truth. Conmen thrive on fantasies and the marks who collectively fall for the con. Their cons are damaged by truthtellers.
But he's doubtless been told it would be disastrous to the response, and hence his re-election, to fire Fauci and so the one tool he likes to wield, he cannot use.
And so he retweets.
It demonstrates his feeling of castrated powerlessness. The virus doesn't care what he says. Or what his marks say when trying not to lose face.