IIRC, the governor can't just name a replacement. They would have to call a special election. This special election would have allowed Thomas Massie to run, and potentially become the incumbent for the midterm election. This is why they have held off on announcing his death for so long

Though, now it is too late to call one, so they have no more reason to keep the charade going. Even if they announce his death, the seat will remain vacant and the midterms will decide who takes the seat.

This is my understanding of the situation.