Because the actual process for him to do so was and is being followed. There are legal requirements to do so and his campaign is doing them. The public part of that is not remotely relevant. It means nothing whatsoever. Harris could, though it would be moronic, have a full press conference claiming she dropped out without actually doing so.
What the little box of lies in your house tells you is not reality. There are processes that must be followed for all of this to actually happen.
It is childish view of the process to think that things are done by press conference. That is just not how the real world works. There are processes set up by each state dictated by law and by procedures set up within the party entity of that state. Biden will have little actual interaction with the process itself other than directing his campaign to do the necessary steps to close out his campaign. If you think he did not direct them to do so is nothing more than conspiracy based on conjecture and ignoring all, and I do mean all, of the actual facts on the ground.