Any argument of the form "you have to write this idiom" is covered by "so either it doesn't happen often, or one can use a macro". There's cognitive overhead to using a macro, but there's also cognitive overhead to remembering car and cdr work on nil. The latter is already paid for, so changing Common Lisp now doesn't make sense, but in an alternate world with a different design it would be a point.
There's more 'cognitive overhead' to making CADADR etc. a macro that expands to the above when CAR and CDR don't work that way, since then its implementation isn't consistent with what it's meant to be. If you made it a macro with a different name then you have two slightly different versions of CADADR and every other accessor, which is even more overhead. Apparently this idiom happened often enough that it was deemed desirable to simply make it the default behaviour. Accommodating common idioms is a pattern of "the right thing" design, from which Lisp is heavily derived, and if you're not a Lisp programmer then keeping this philosophy in mind is a good way to not have misconceptions about the design of the system.
However, thinking in terms of 'cognitive overhead' for a very minor design choice is very silly. I don't suffer any 'cognitive overhead' from having CAR and CDR work on NIL when I write Common Lisp because I'm used to it, but I do suffer 'cognitive overhead' when they don't in Scheme, which is the 'alternate world with a different design'. I am incredulous to the idea that one is actually superior to the other, and suppose that it is simply a matter of preference.
