"Why drop out the day before Super Tuesday?"
I think the democrat party leadership cut deals with Amy and Pete. Pete won't be elected for any Statewide office in his home State of Indiana. He's likely been offered something in Biden's cabinet. Amy might even be looked at as VP. She'd pull in the women's vote (and she doesn't have the ABC poison -- Anybody But Clinton), and what a great spot to find yourself in after the election -- VP for a 77 year old President with dementia. If Biden doesn't die, they'll probably use the 25th Amendment against him.