These are mostly idiots and morons. Not saying that as an insult. That is truly exactly what they are. They do not know anything about anything. Therefore they just randomly pick 'Biden' not knowing what or who he is. Or, they just picked a name they heard on CNN/MSNBC and have been told that is who they are supposed to pick.
Also, if the poll presents Trump vs. Biden, many of the intellectually challenged will pick the "Not Trump" choice. It is not that they 'support' Joe Biden, they have just been inculcated with the idea that Trump is Satan on Earth so they reflexively pick "the Other Guy" whoever that may be. In this case, it just happens to be Biden. If they replace 'Biden' with Hannibal Lector, then Hannibal Lector would get 30% support.
All of the polls showing 30% Biden support does not mean 30% support him or even know who he is. It is more reflective of name recognition and MSM conditioning than it is a considered opinion.
Same on the other side. In a fair fight, Donald Trump would be at the bottom of polling for the GOP nomination. But people recognize his name and have conditioned themselves to refuse to consider anyone else.
I'm THIS close to running a longshot third party bid myself.