It's fine to say you don't know. But you're responding to people like you do by pointing at a result as the reason for...the result. So unless you experience time non-linearly, it's a bit odd.
I am pointing to the result in an attempt to get people to take a step back and reassess.
So going into detail on why policy doesn't always matter. A few reason's:
People who have been around a while have seen a million places come and go, they have even seen them come back around again on a cycle. Ideas have value, but they aren't everything and people have become pretty cynical about political promises in general.
For many voters, first, before they even consider a person's policies, they are going to judge the person's character. This is sort of ephemeral and it is a very emotional thing. (It can also be a very bad thing because it allows bigotry and bias to drive political support.)
So that's a pretty big problem for politicians; it's a tough thing to break through that cynicism.
One of the ways you do that is by being around, being present. By making people think that you are there to listen to them rather than being there to preach at them. Biden has cultivated the image of being someone who does that. (I want to stress "image" here, the perception is what matters).
Another thing that is helpful is to have a track record of accomplishing things. This shows that you have some level of clout, some ability to deliver. Biden has also spent a lot of time making this argument. Getting an endorsement like Clyburn's is a massive boon. It carries with it a huge amount of inherited trust.
It's about making people feel seen and secure. A campaign that is able to do that is doing some things right. And none of those things really need to involve getting deep into policy.