It’s because it’s from the French (if you say the plural with a French accent, it all makes sense). There are a handful of other cases in English, though none spring to mind this morning.
I feel like you’re half right? Daughters in law and passers by don’t feel Norman French. Daughter is definitely from middle German, yeah? And I am wondering about the professors example because the whole point is the plural only happens on the first word.
That said, I so want to be a part of the last group.
