Because several parts of the Bible do contain reference to 'Scripture', so 2 Timothy 3:16 (KJV) tells us:
So what is "scripture" to "Paul"? Only those books deemed fit for the canon, to be decided several hundred years hence??? Only the most naive of Christian apologetics can claim that 2 Timothy 3:16 is a reference to the Bible - despite the abject naivity, I do appreciate that it is often done.