Democrats and the Biden campaign would much rather talk about Joe Biden's age and mental acuity than defend his being complicit in a genocide in Gaza, than his handling of the withdrawal from Afghanistan, than the fact he lied about not leasing public lands to oil companies, than he actually leased public lands at a faster rate than Trump did, that he's kept and even expanded Trump immigration policies that he ran for president opposing, that he backed the Willow Project in Alaska and another pipeline project, that he failed to deliver on student loan relief, that he totally ditched his commitment to expanding access to childcare the moment he got elected.
It's much easier to defend his age and mental fitness than it is to try to defend the total and abject failure that is his presidency