I'm not disregarding Biden's accomplishments.
Let's be honest, Phan; when Biden ran in 2020 it was 1) with the knowledge that he would be a one-term President (due to age) with a single mission: Beat Trump and 2) he was the only candidate the Democrats had that had the best chance of beating Trump.
The Democratic Party clown car of 2020 was full of nutjobs or people who, rightly or wrongly*, wouldn't be able to carry a majority across the country. Four years later, the Democrats are in the same boat; they don't have anyone that can beat Trump except Biden.
The fact Trump is a traitor, likely soon-to-be-a-convicted-felon and completely unfit to be President isn't adversely affecting his chances of becoming the Republican nominee. If Trump wasn't the nominee, would the Democrats really be running Biden again? I doubt it but, at this rate, it's likely Biden will become the Democratic nominee.
*mostly wrongly