You just don't see them because people never sell them. The Atari vectors are permanent features in the gamerooms of most serious collectors. Everytime I go do a deal with someone and check out an awesome gameroom I always see Atari vectors. Asteroids, Tempest and Star Wars mostly, but I also see a lot of Battlezone, Ast Delux, Space Duel and have even spied a few Gravitars, Black Widows and Lunar Landers. Never seen a Quantum though, although I have seen several converted Quantum cabinets.
On the topic of converted cabinets, the I-Robot rarity story is total BS. If there were only 500 of them in the US, then I must have seen 400 of them converted to Choplifter!