Because apparently rhetorically wondering aloud if something like that could be done is exactly the same as telling people to do something.
Except in no world except the hyper-partisan, far-left world of #NeverTrumpers could anybody believe that nonsense. First, the areas initially hit hardest were far-left enclaves where, if Trump said "wear a mask!", most citizens would decry him as fascist and do the OPPOSITE on general principle. Second, it presupposes that a president has the powers of a king rather the elected leader of a democratic-republic. If Trump mandated stay-at-home, masks, etc, almost every governor in the country would block him and many of the first would be the liberal states whining about him now... and his orders would almost certainly be reversed by the courts. Third, previous administrations had depleted (and not replaced) the supply of PPE, so Trump was faced with a problem largely created by administrations like the Obama/Biden one which makes Biden's criticisms of unpreparedness all the more ridiculous.
I might agree - not to back trump or anything, but if "Obama caused Hurricane Katrina" then I certainly can say Trump caused covid