Obama can't be VP either as the rules are that anyone ineligible to be president can't be elected as VP (there's apparently some debate over whether they could become VP via being promoted from lower down in the succession in the event of a presidential vacancy).
Sanders could, but I think he has the double issue of his two prior failed campaigns - a lot of his more motivated voters from previous times might have a "boy who cried wolf" reaction a third campaign. He also doesn't really disarm the "too old" criticism, even if all evidence points to him aging a lot better than either Biden or Trump so far - Biden 2020 vs Biden 2024 would give a lot of people pause even if Sanders gave the world's best debate performance at this point.
There's also the fact that Sanders has been actually pretty influential in Biden's presidency, which I think is why he came out so strongly in favour of Biden a few days ago.
It's not obvious how much of this is Sanders' impact on Biden vs. Sanders' impact on the party as a whole, but Biden's pro-union and pro-worker stances are pretty pronounced compared to Obama and Clinton (and remarkably so compared to Trump, and Bush). So he's easily the most pro-worker/union president in my lifetime, which is a fairly Bernie-ist position.
Same with (partial) student debt relief, and you see shadows of the Green New Deal in the Inflation Reduction Act.
Sanders could, but I think he has the double issue of his two prior failed campaigns - a lot of his more motivated voters from previous times might have a "boy who cried wolf" reaction a third campaign. He also doesn't really disarm the "too old" criticism, even if all evidence points to him aging a lot better than either Biden or Trump so far - Biden 2020 vs Biden 2024 would give a lot of people pause even if Sanders gave the world's best debate performance at this point.
There's also the fact that Sanders has been actually pretty influential in Biden's presidency, which I think is why he came out so strongly in favour of Biden a few days ago.