It's possible Biden's COVID is worse than being stated, and that's the reason we aren't seeing him.

They could be doing what the Dodgers did to Tommy Lasorda in 1996. He had been the Dodgers' manager since 1976, and was a beloved figure to fans, so he was very difficult to fire. However, he was almost 69, making an increasing number of bad in-game decisions, and team ownership really wanted him out. But he wasn't just someone you could fire without a huge fan backlash.

Then it happened. Lasorda had a heart attack. He recovered and his life wasn't in that much danger, but this was the event they used to get him out. They claimed that he needed to take some time away from managing to recover, and appointed Bill Russell as his "interim" replacement. A few weeks later they let him know it was permanent, but he wasn't being fired. He was instead turned into a "vice president" of the team, whose position was mostly ceremonial/promotional. Aside from a quick stint as GM in 1998, he remained a meaningless VP until his 2021 death.

The jackals at the DNC probably used Biden's COVID to convince him that he was too frail to continue the rigors of a tough Presidential campaign, and browbeat him into agreeing to step aside. I'm guessing they are waiting until he recovers to present him to the public. Until then, he will be hidden in the basement as usual.