Because they said he had mild symptoms, some other doctor in The White House said that and also The White House Press Secretary said it.
They all have reasons to downplay his condition. I understand why. A debilitated President is not good for the country.
Regeneron is in trial. It's one of the hoped-for vaccines Trump has been pushing to get approved. Thus far, it seems to be working pretty well. In trials. It may fail, I don't know.
But there can be other reasons why he was rushed to Walter Reed.
It's possible that he really was exhibiting mild symptoms then suddenly started exhibiting more serious symptoms.
But this is a President most Americans think isn't truthful with the public. So you shouldn't necessarily believe that what comes out of the White House is the gospel truth.
I'd say that about any President for my first sentence.