I can't find the article at the moment, but I read somewhere that the reason they did a video announcement is because they didn't want Biden announcing in person and screwing it up. Supposedly, he will be doing few live appearances because his aides don't want the public to see that he is in bad shape. They are intent on defeating Trump and don't care about the possibility that Biden won't be able to serve a full term and Harris will have to be sworn in despite her obvious incompetence.