I don't really have anything to support that in-game, but tank weapons could be somethinmg similar to today's unmanned turrets. These are basically self-contained weapons installed on top of a vehicle, containing ammo, ATGMs, fire control, comms, drones, etc.
Still doesn't make sense why aircraft can't use 'beam pods', which is a very similar concept. But, as I said, I kinda like the implications of this, including having to design and deploy new platforms for actually using your shiny Wunderwaffen.