Why? Is President Trump opposed to "smoking out the Resistance"?
No, it’s just more dramatic and more powerful if you don’t know exactly who’s writing it.
Assume it’s Mulvaney, which I have no idea whether it is or not, but just assume it is. Then it’s easy enough to just say, well this is Mulvaney being Mulvaney.
But what if it’s one of the more normal, somewhat sane cabinet officials? Then it could be a bigger deal. So, best to keep it anonymous. There’s a lot of administration officials who are not in fact particularly influential in ANYTHING, so their anonymous opinion is more newsworthy than their signed one.