This is the problem he had in 2016.
He could have wasted Hillary if he started punching. Understandably I can see him not wanting to accidentally hang himself with rope made out of tissue paper after slipping on a bar of soap at some later point.
But Biden? Sanders could lay into him like it was going out of style, and he wont.
He got paid off by the DNC to go away and got a new lakeside vacation home in the process.