I have the same issue as you. And I'm not asked if I want to apply this function to other cursors ... strange, becuase I did expect this question. Maybe there are some changes in multiple-cursors recently?
I also can't answer your "why" question.
But here is some source code, to make it do, what you expect.
(defun replace-last-sexp--internal ()
The repository README.md -- https://github.com/magnars/multiple-cursors.el -- contains a basic usage introduction that states in relevant part:
"When you have an active region that spans multiple lines, the following will add a cursor to each line:"
(global-set-key (kbd "C-S-c C-S-c") 'mc/edit-lines)
Mark many occurrences
mc/edit-lines: "Adds one cursor ...