Or maybe the 2 just understand that they can't win and won't risk the same as the GOP 4 years ago when internal fighting really helped Trump?
Sure some forces in the party (and very possibly Biden's campaign as well) has most likely pushed for it but really?
Bernie is an outsider who only sees himself as a democrat when running for president. It's part of the mechanism to be wary of such and really not of the first worries for the US democracy.
Don't get me wrong, the DNC are nowhere near great democrats, but the drop-outs aren't really that unusual to a democratic movement.