Its not like Biden has the pick of the litter.
When you have such an uphill chance of beating a popular incumbent President, usually the first tier VP choices are not interested.
They usually have to settle for a schlub who is fine with also ran status.
It always baffles me why Hillary picked such an unattractive nobody like Kaine as her VP. If she would have gone with a Michigan, Pennsylvania, Ohio, Iowa, Florida or Wisconsin person, she would have won (despite how wonderful Trump is or how masterful a campaign he ran).
She could have picked the best VP in the country and it would not have stopped how bad a candidate she was. You cannot manufacture charisma and she had none.
She was simply the worst candidate ever chosen until now.