Zeta is the sixth letter of the Greek alphabet, not the last, which is omega. The Romans kicked it out of the Western Greek alphabet, and then had to bring it back in along with K and Y when they started using Greek loanwords.
Zeta was the sixth letter in the Ionic alphabet, which eventually became the standard alphabet, but some early regional alphabets had the letter digamma (which looked like the Roman F and was in the same spot). In the Greek numerical system, zeta equaled 7 (digamma was 6).
With Greek numerals, the first 9 letters were the numbers from 1 to 9, the second 9 stood for 20, 30, etc., and the third 9 stood for the hundreds. That required using three obsolete letters--digamma, qoppa (equivalent to our Q, immediately following pi), and sampi (a sibilant, following omega). The number 111 would be represented with rho-iota-alpha. It makes the Roman numerals seem easy.