Tombolos (accent on the "TOM") form as an offshore hill, or stack, bends incoming waves around it so that their energy sweeps sand together from both sides. Once the stack erodes down to the waterline, the tombolo will disappear. Stacks don't last long, and that's why tombolos are uncommon.
See this article for more about tombolos, and see this gallery for more pictures of tombolos.