Deploying vehicles on the battlefield would substantially increase our firepower, but we're facing severe space and weight constraints, hence the vehicle has to be small, with a very tight cockpit (especially for us, Ubers). Crafts can take only as many vehicles as they fit, and even then, each vehicle counts as 4 soldiers. Production and storage of vehicles takes lots of space. A final note: vehicle weapons might need to be manually loaded at deployment (SHIFT+LMB to unload).