The alternative to trying to get them to learn is essentially giving up, at least for the foreseeable future, so we have to try something.
There are tons of reasons why someone might figure it out now, too:
They're personally doing worse now than they were before.
They weren't alive/politically active before.
They reach their limit on trying the same thing and getting the same results.
They see a growing left wing of the Democratic Party that hasn't really been there since the 70s.
They see a few good things happening (improved rights/representation for the LGBT community, serious steps to winding down the war on drugs, the spike in union activity) and figure out that the libs saying those were politically infeasible are full of shit.
Someone close to them figures it out and slowly brings them along.
If they didn't learn from Carter, Clinton, or Obama, why would they learn from Biden?
The alternative to trying to get them to learn is essentially giving up, at least for the foreseeable future, so we have to try something.
There are tons of reasons why someone might figure it out now, too:
Also, Biden is markedly less popular than Clinton or Obama with the libs