It could be said to have been an abbreviation for so long that it has developed a life of its own!
PS - "its" as possession is the exception to my previous post, but "it's" as abbreviation for "it is" does have the ' for the omitted letters. The English language is full of such things!
Prayer ban at Katharine Birbalsingh’s school is lawful, High Court rules .
France imposing a fine for no-shows at GP surgeries.