Why CQ is the universal call meaning "I am seeking a contact with anyone who hears me." It is a general call, not addressed to a particular station, so any station hearing it may reply. Directed forms exist too, like "CQ DX" for distant stations or "CQ contest," but the base meaning is a call to any station.
Watch out "Contest QSO" is a made-up expansion; CQ predates and is independent of contesting, and "CQ contest" simply narrows the general call to contest participants.
CQ sounds like "seek you" - seeking anyone who will answer.