Who could he pick that would help Trump? And likewise who could he pick to help himself?
Normally, I don't think the VP matters much.
However, there are legit concerns about Biden's ability to finish a term, so it might.
Nobody likes Harris, which was evident by the polling in her own party's primaries, and she's not going to convert any independents who voted Trump last time.
Maybe Klobuchar, since the only qualification Biden's said is necessary for his VP is that she has a vagina.