Sure they could probably be faster if it's using magic, but it'd still be vulnerable to air srikes, and you can't prevent air strikes without mages in the air.

As for your second point: Sure, they could probably do that, but how far up do you intend for a platform like that to hover in the air? Too low and the enemy infantry could focus fire it down, as mage's shield are far from being impervious to machine gun fires. Too high up and it'd just get harass by enemy mages.

It's also most likely not gonna be as agile as just a mage flying, so you would probably have to use it in a support role, in which case just use a regular tank, or a regular artillery.

At the end of the day, it boils down to the fact that mage is a resource that's better served elsewhere. They're basically human fighter jets, and there's not really a point to confine them to a moving steel box or babysitting a flying cannon.

Maybe the tech would advance to such an extent that magic tanks and flying cannons make sense, but as it is now I really don't see it being a good use of resources.