With a pity, a shame, or similar disappointments, they are countable up to one.
Or you could see them as "fixed phrases" which are invariable whether or not they currently obey the norms of speech. "Aren't I" is such a fixed phrase.
We must not see living languages and their syntax as strict, well-defined computer programs without exceptions; they are as full of inconsistencies as we human beings are.
"... Our frailty is the cause, not we
For such as we are made of, such we be."