It's a bad thing to compromise ones core values for a career. Unless that person doesn't genuinely hold them. Then it's just about power.
Perhaps Harris sees that her core values can be upheld as VP under Biden, esp. if it opens the door to a presidential run when he leaves office? I see her as a possibility for a strong, active VP rather than a passive mouthpiece VP. And, after all, there's a non-zero chance this guy will be a one-termer given his age.