Why do we have 1&2 Samuel, 1&2 Kings, 1&2 Chronicles?

Old Testament Hebrew was written without vowels. When the OT was translated into Greek, the length of the content increased greatly because it included both consonants and vowels. This meant that what previously fit on one scroll now had to be written on two scrolls, necessitating the division of the books. Our English Bibles tend to follow the Greek manuscripts, so we inherited divided books.