I'm struggling as well. Only other theory I have is Biden hid it from them as well, otherwise he might have had real opponents in the primaries .
otherwise your back to echo chamber theories (as in dems echo chamber saying he's fine)
Biden couldn't hide from us, but could he have hidden it from his closest allies? That doesn't seem plausible.
My only explanation at this point is that they believed replacing Biden during the primaries would have undermined the remainder of his presidency, potentially boosting Trump's chances. It's also possible they couldn't convince Biden to go along with such a plan. Their next best option may have been to make a late swap that could generate momentum without giving the new candidate enough time to falter.
Another viable strategy could have been to keep Biden but replace Kamala, but that might have alienated their leftist base.
Alternatively, it's possible that they are not as clever, united, or fortunate as they portray themselves to be.