She wasn't appointed. She won the vice presidency as part of the Biden/Harris ticket.

At the convention, the selection process would be one dominated by mega donors. No candidate will survive a first round, which will leave super delegates with the choice. The money would win.

Harris was chosen as VP. Part of that means that if something happens to Joe, she steps in.

We actually did already vote her in. She's been vetted. There is nobody else available with sufficient name recognition who can win.

3/4 of the country doesn't know who any of the other possibilities are. Worse, they haven't been vetted in a national election.