Why did Sumerians carve in to tablets instead of using ink like the Egyptians?

Reeds were available in plenty in the Nile Delta, so the Egyptians used papyrus and ink. Clay was plentiful in the land between the Tigris and the Euphrates, so the Sumerians used clay tablets and stylus'. Sumerians are believed to be the older of the two civilisations, so before they discovered parchment or paper they used clay. Bear in mind, the Egyptians hieroglyphs are carved too on the sand stones.