@RandomPhoenix
The thing that gives them magic powers is called orbs for future context.
Efficiency and cost:
Mages act more like helicopters due to their low speeds, altitude. They are also cheaper to assemble than building aircraft, which takes more time, engineers, and resources than slapping that orb on, putting on your uniform and equipment, and GTFO.
Base mage advantages:
For mages under 6000ft vs. other ground-based infantry below that altitude is basically prey for the average mage. They are silent compared to aircraft and are more accurate. They are a smaller airborne target, which is highly beneficial in being an artillery observer/reconnaissance as well as bobbing and weaving anti-air artillery, they already have the firepower of artillery (they have a spell literally called artillery), they have better maneuverability than aircraft, and overall they act more like special forces than a stand-in version for fighter planes.
Mages probably cant power a vehicle:
Mages are a new technology; they can channel their magic into their weapons, sure. But you're talking about doing the same thing into a 28ft. 2700 kg vehicle, that's an entirely different story. The average orb can barely provide a mage with enough power to reach past 6000ft in altitude and still cast combat spells. Mages use the orb to channel their magic into reality; it does not channel magic independently. That means if a mage powers themselves using an orb and can barely reach an altitude of 6000ft, powering an aircraft bigger than them would put a massive strain on their body, in theory. In fact, a mage would spend more concentration and magic trying to power an aircraft than they would need to if they were to fly independently.
Bigger orbs = going broke:
There is also the economic viewpoint; The average orb is very expensive and costs the same amount as a flagship or aircraft, and it's still the size of a pocket watch. Even if that mad scientist somehow made it big enough to suit a plane, even if a mage was able to channel that amount of power, the sheer cost of that orb would shoot up at least 5 times the amount of its pocket watch base size. It's way too expensive, and it causes the mage to miss out on the advantages I've listed above. This could even be a big factor in why an orb isn't made bigger; it's too expensive. And when it comes to war, spending can be great for the economy, but spending too much can leave a country in an economic crisis. That would mean they can't produce a lot, and if they cant produce a lot, then what's the point?
Even if a mage didn't power an aircraft and just used the base Bf 109, Imma keeps it real with you the aircraft of less than the 1950-the 1940s are just straight-up trash. They don't even compare to the accuracy, maneuverability, and firepower a mage has. The only pro they have is their altitude cap, which wouldn't even mean much because we seen aircraft get snipped by a mage's magic beam.
Now, looking back at everything, I have way too much free time.