ZAX units are advanced supercomputers that are capable of learning and are considered to be the first real A.I. in the fallout universe. The only times we have seen them ( the 2 times) used have been for scientific research or monitoring for the military. Some, as in the case of John Henry Eden, are also self-aware. The first unit was made in 2059, but so far we have only seen 2, ZAX 1.2 and John Henry Eden. Considering the ZAX units existed 17 years before the war shouldn't have seen more? It seems like something this much processing power and ability would have extremely useful for both vault-tec to monitor their experiments, and for the military for planning as ZAX 1.2 is unbeatable at chess even for character with 10 intelligence.

Also, it's not cannon but there were 3 units planned for cancelled fallout 3.