Just dumb. Ever since the first time I watched Biden on TV, back in the early 70’s I knew he was incredibly stupid and have been forever mystified that few seem to also notice. Mind you, I’ve known and worked with a few extremely (and inexplicably) successful morons. So in that respect he’s hardly unique. Rep. Occasionally Coherent is just a more recent and extreme example.
It is said that Obama selected Biden as Impeachment Insurance.