Your theories about what can and can not be done with ordnance,
Not theories, practicalities. Field artillery is designed to be fired up and over long ranges and over obstacles. The trajectories that it utilises are designed to allow it's rounds to be fired out to the maximum range of the weapon and over intervening obstacles. Anti-Aircraft weapons/Anti-Tank weapons/Coast Defence weapons are designed to be fired in straight lines, directly towards the target. The only similarities between the types of weapons is that they both go, "BANG!"
The utility of field artillery is such that the crew must be able to readily alter the amount of propellant in the chamber. This is done through having small bags (referred to as "charges") of powder which can be easily removed or added to, when the round is loaded into the weapon. While humans can do this easily, machinery finds it a lot harder. This can be helped by making the charges into small packages in cardboard or plastic. However it still makes it fiddly.
Anti-aircraft/Anti-tank/Coast Defence weapons use fixed rounds to enable easier loading faster. They have fixed charges in the cartridge case. This limits the trajectories they can undertake but then, they usually aren't required to fire indirectly (although, towards the end of WWII, AA guns were used in indirect fire missions - it basically gave them something to do when there were so few aircraft around).