At the same time, Trump is all over the media, every day, for the last 5 years, campaigning constantly, earning votes making an ass of himself.
FIFY
Trump had the luxury of Obama's economic coattail on which to ride for the first three years of his tenure. That coattail is gone, and Trump has been doing nothing but floundering, flailing, and making a general ass of himself ever since. He has splintered the GOP and caused a significant group of them known as the Lincoln Project to actually go out and announce their support for Biden. While Trump is committing political suicide, what has Biden been doing, you ask? The smartest thing he possibly could be doing right now: Keeping his mouth shut and letting Trump do all the work.
There's still three months between now and election day. There is no hurry at the moment to go stumping on the campaign trail. Everyone knows who Biden is, so that works in his favor. He doesn't need to spend time introducing himself to the country. And quite frankly, the less he speaks before the election, the less ammunition he potentially hands to the Trump campaign.
Democrats, and even a handful of Republicans, see one campaign issue, and one campaign issue only: Get rid of the Trumpasaurus Rex. One doesn't need to do very much campaigning on those grounds. And most of us Democrats fully realize that most, if not all, of Biden's first term is going to be spent undoing all the damage Trump has done. There will be time enough to get down to the brass tacks of politics and deeper issues once that's done. When your house has done burned down to the ground and you are in need of shelter, you don't go shopping for wall decor.
ETA: Congratulations. You have actually made me write what is probably the longest post I've written since VVO came online. I'm usually known for my brevity.