I'm genuinely unsure if they are just so immersed in the group think that the only way to win is to move rightward and being seen as liberal is a death sentence or if they would just prefer to lose than move left.

I think some of it is selection. Candidates who have centrist instincts are rewarded with donor money and media support, liberals and progressives do not receive donor money and media attention, if they're not actively sabotaged by same. Biden was pulled out of retirement by media and Republican donors as part of the anyone but Bernie movement.

Whether he is beholden to them or just shares their allegiance to centrism, he will continue to move to the center chasing imaginary disaffected Republicans and ignore his base come hell or highwater.