I think most/all accouting firms are privately held
I'm not sure about AA, but when I was with KPMG, they were a limited liability partnership.
I believe the idea is that if they're doing the books for a firm, they can't be unduly influenced by said firm suddenly purchasing controlling interest through publically traded stocks.
Based on recent events, this particular safeguard doesn't seem to work that well, though....
Tom Sinclair
Speaker-to-Suits
Natural selection saw to it that professional heroes who at a crucial
moment tended to ask themselves questions like "What is my purpose in
life?" very quickly lacked both.
-- (Terry Pratchett, Interesting Times)