Brown is only 75, and even though he was never popular with the voters, he's fairly well liked in the party. Maybe that's part of the calculation.
Tom Harris recently wrote an account of Phil Woolas's funeral - someone else who wasn't a popular hero with voters but was liked by party insiders - and apparently it was a reunion for New Labour veterans. Blair and Brown and Kinnock were all there, hugging comrades and swapping old war stories.
Keir and Victoria turned up, for why nobody knows because it seems neither of them knew Phil Woolas or even met him, and sat there not talking to anyone because they didn't know anyone.
Those kind of deep connections matter. Even a youngish man like Wes Streeting has loads of personal relationships in the party going back to his NUS days. Starmer doesn't, and maybe he sees Brown and Harman as his way of acquiring some sort of credibility with party stalwarts, in his usual clunky tin-eared way.
I mean John Major (83) has never really retired, he's still extremely active trying to pull strings in Tory factional battles. Kemi is IMO far too deferential to Tory grandees, but even she hasn't been daft enough to bring Major into her top team and trumpet that as building for the future.