If you ever looked at the portraits of the Factory units in Starcraft II, they tend to look less ‘military’ and more ‘unprofessional’ from Starcraft/Brood War to Starcraft II (e.g the Siege Tank driver in SCII looks a lot like a mechanic) . For the Raiders, they are disgruntled civilians that were recruited by them who have previous experience with operating vehicles and hence became vehicle crewmen. But what about the Dominion ? Well, for the Dominion, there are two fates for every criminal sentenced to neural resoc, becoming a marine or becoming a vehicle crewer with new skills to operate the vehicles in the Terran Dominion’s military granted to them by the neural resocialization process.
(This can be used to explain the Hellion/Hellbat, Cyclone and Thor pilots in the unit portraits, they were originally criminals who were sentenced to serve as vehicle crewers. )