For Obama it is all about Barry anyway and he is in a win/win situation and there is no real gain in getting involved. If the dems win, well he wins because all his buddies are back in power and welcome may is down for him in every square inch of DC. But if they lose, well then he is still treated as a demi-god by dems - the last clear example of someone who actually looked and acted like a dem candidate or president- rather than the old white geyser squad they have now.
The other little factor- that Obama is well aware of and dem strategists as well- is that Obama's record of endorsing and "helping" other candidates by campaigning with them is actually atrocious. Obama himself has said that in many interviews since leaving office. That although he was able to get elected and draw crowds that he was unable to actually help others. Of course, this is just Barry letting you know again that although he himself is awesome, he cannot make anyone else awesome. So his track record is not good there, and endorsing Joe would not help it at all. It would just be a chance to come back on to the scene and demonstrate that, not only does he not hold much sway, but that the progressives have issues with him that they have agreed to not rub in his face if he stays out. We have seen little anti-Obama episodes flare up that had to be tamped down, eh?
let's not forget Trump's bad record in endorsing candidates in 2018 and I did not see Bush campaigning hard for McCain. No one has a good track record in that respect which is why they dont or shouldnt do it...