They don’t actually have the same sound—or at least, they didn’t use to, in pre-Exile times. The tet is supposed to be pharyngealized (pronounced from the throat), while tav is not (some even speculate that tav may even have been pronounced like the English /th/).
However, in recognition of the fact that they are similar, however, when the Canaanites invented the alphabet using simplified versions of Egyptian hieroglyphs, they made tet a derivative of the tav in its form. Tav in the form of an x- or “+”-like sign, as can be seen here in a grafitti inscription made on an Egyptian statue in the Sinai ca. 1550 BCE:
(from:, where it’s shown upside down)