Its not firing up enough of the electorate from different groups. Those that vote Republican love Trump - he has solid support and just needs some of the center to win.
Biden is not seen as progressive by Bernie voters and as the strategy has been to keep Biden out of the public eye (I guess in the hope that Trump will keep making major blunders) voters are not sure who he is, other than not Trump.
There isn't a strong exciting Democrat challenger and many people feel unrepresented by either of them.
It depends what your issues are as a voter.
It makes no difference how many extra votes Biden gets in traditional Democrat strongholds like the Pacific coast states and NY, its places like Florida and Texas that matter.
Everyone knows Trump is a jerk, voters clearly aren't choosing him for his character. He is acting like a Teflon bulldozer that is clearing the way for the policies that GOP voters want. My in-laws are against transgenderism, pro-military (they both were in the military and then in social services) pro traditional family, God & country type people. They are very involved at their church. They help a lot of people practically like a single mom they know and her 3 boys. I think they don't really understand what the Democrats are offering at the moment other than wanting to tear down American traditions.
I think they will very much want Trump to put a conservative judge on the Supreme Court before the election.
I can't even vote as I am not an American citizen.