Because, whether he says so aloud or not, the best outcome for Biden and the country would be a one-term presidency that restores decency to the White House and faith to U.S. alliances, and then gives way to a new generation. The person nominated for vice president in Milwaukee in July instantly becomes the front-runner in 2024.
Two consequences flow from that.
The veep choice is sometimes thought of as a way to balance the ticket ideologically and heal a party. But if Biden, or Sen. Bernie Sanders (I-Vt.) for that matter, essentially anointed someone from the wing of the party that opposes him, the depth of his own convictions would be immediately suspect.
Second, the choice should, and almost surely will, be a woman.