jets are nearly useless against a flying human target , it's too small , jets weapons have no auto-tracking , flies too fast to be able to be as agile as a mage , fuel consumption is HUGE so it can't stay in fight for long , it's fast and difficult to aim at but mages have auto-tracking so it's not an issue as long as they're lower in altitude.
and if we ever see jets in this manga , it will be the first jets ever so it means it won't even be that good.
the only good thing they'll do is basically bomb the heart of the enemy territory , mages shouldn't be able to do anything against that because the jets would be able to attack very fast from a high altitude before the radars can spot them and the HQ can dispatch mages.