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: Proto-Sinaitic script – Wikipedia, where it’s shown upside down)
The letter tet was the same, but with a circle around it:
As the Canaanite evolved into the Aramaic script, the circle of the tet became the dominant element, and the “x” within it reduced to just a residual diagonal:
Notes by Autumn Light 
My own explanation (laid out at length somewhere way back when in WP archives) is that the ‘Children of Israel’, ( as we lucky ones who read the Bible in the original English call them), saved money when they needed to purchase an alphabet; requesting a ‘bare-bones’ (partial skeleton?) set. Compounding the sad tale was their unwise choice of an unknown young ‘purchasing-agent’, who spent half of the purchase-money early in the journey on.. on something, lost to history. He returned with the few letters remaining after pawning off several for bus money. And finally tet and tav are really the same letter, simply ‘altered by the kid’s pocketknife on the bus. As are caf and koof, heh and chet, ayin and aleph, vav and vet.
And now we be stuck wid it. Hope this clears things up…
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