I feel this is entirely context dependent.

But here are a bunch of reasons way

Mages are powerful, thus don't take orders from schmucks

The mages are so powerful that they can do what they want. So. You don't simply drag a mage and throw him into the army. The first time a drill instructor insults him the drill instructor gets incinerated. Similarly you can't command them. They are powerful and don't take kindly to be ordered around. They also have little incentive to join in the first place. Context because you can argue making measures against that but still. Unless you chain them all the time and only use them at the moment of battle then they will snap at you.

Also the chain them up option is hard because how can you figure out how to stop their magic without magical help? And even if you do it then you have a lot of angry people with superpowers who are just waiting to rise against you.

The French revolution or Bolshevik revolution aint got nothing on that of a revolt of mages.

So. In that world mages are rarer than normal people. But battlemages are even more rare. This means that while you can relay on a single battlemage then the other nations of the world can't.

Also in many worlds magic seems to be diverse and not a single field. Which only adds to my point. For example you can have mages be very powerful but the ration of the battlemages to wizards is the same ration of wizards to normal people.

If you compare this to say full plated knights then you can get a better understanding. Nobody just went around with 50000 full plated knights. Sure they are almost a superpower but that destroys the dreaded budget.

Yes knights and men at arms are different from mages but I want to give an example of something close not exact.

Context right of the bat.

But if certain settings have magic come from a hellish dimension why cant we have it come from a purity dimension?

Magic here is all about building and healing and restoring...etc

There might be sever penalties on doing anything else or trying to find a loophole. Maybe angels or the magic itself. Maybe corruption takes hold of those who try to twist magic.

I mean you can cultivate growing by prearranging the woods of a tree so they grow and prosper only first punctuating the bodies of an opposing army.

You can heal people by increasing their bodily functions say the heart making it bumps several hundreds time faster to also kill them.

So. Loopholes can be made. But maybe the system is watched over by something or it is half sentient itself.

This can easier be that mages require such high wages then hiring an entire army is cheaper and gets the same result. Or that magic itself requires some rare metal or liquid or crystal or stones or whatever else happens in the world. Again context

Again context dependent. But maybe the world in question has powerful magic that comes at the cost of demon pact or hell portal or mages exploding...etc

This setting is not undone and most of the time magic is still practiced but it would actually give generals pause. You honestly don't want your mage to fall to demons mid battle and open a portal to hell from which a bunch of demons spell to destroy your army. If you can control that magic then you throw it at the enemy. But that is a one time use thing. and again context.

This is pretty much the story of 95% of weapons programs and how they do before they are born. Weapon X is promising such improvement on the rifle of the army that the army is interested. After a round of testing weapon X is thrown into the trash because it is unreliable. Soldiers hate anything unreliable and prefer any reduction of anything except reliability.

Well. The praetorian guards literally auctioned the imperial throne of Rome at some point.

Also from Rome. After the Marian reforms the army, usually more than one legion, started to have a very strong bond to the general. Que the civil wars and we have the armies deciding who is the next emperor by simply backing them up and marching on Rome.

So. The people of your world figured out that it is a bad idea to even recruit mages.

See. You have 10000 soldiers who are fighting. A mages steps out of his tents and rains fire on the enemy, he also heals the wounded soldiers. He inspires them and might even destroy an entire fortified city with a snap.

How long till that guy rises to be a warlord?

History tells us that without magic this is always the case and a big risk.

With magic it is amplified a billion times. People follow charismatic leaders to death all the time in real life. So. Context

I'm pretty sure I can go on and on. So. Again. Context

I feel this is entirely context-dependent.

But here are a bunch of reasons why:

Mages are powerful, so they don't take orders from schmucks

The mages are so powerful that they can do what they want. So, you don't simply drag a mage and throw him into the army. The first time a drill instructor insults him the drill instructor gets incinerated. Similarly you can't command them. They are powerful and don't take kindly to be ordered around. They also have little incentive to join in the first place. Context because you can argue making measures against that but still. Unless you chain them all the time and only use them at the moment of battle, they will snap at you.

Also, the chain them up option is hard because how can you figure out how to stop their magic without magical help? And even if you do it then you have a lot of angry people with superpowers who are just waiting to rise against you.

The French revolution or Bolshevik revolution ain't got nothing on that of a revolt of mages.

So, in that world mages are rarer than normal people. But battlemages are even more rare. This means that while you can relay on a single battlemage, maybe the other nations of the world can't.

Also, in many worlds, magic seems to be diverse and not limited to a single field. Which only adds to my point. For example, you can have mages be very powerful, but the ratio of the battlemages to wizards is the same ratio of wizards to normal people.

If you compare this to say full-plated knights, then you can get a better understanding. Nobody just went around with 50000 full-plated knights. Sure they are almost a superpower but that destroys the dreaded budget.

Yes, knights and men at arms are different from mages but I want to give an example of something close but not exact.

Context right off the bat.

But if certain settings have magic come from a hellish dimension why can't we have it come from a purity dimension?

Magic here is all about building and healing and restoring... etc.

There might be severe penalties for doing anything else or trying to find a loophole. Maybe angels or the magic itself. Maybe corruption takes hold of those who try to twist magic.

I mean, you can cultivate growing by prearranging the woods of a tree so they grow and prosper only first punctuating the bodies of an opposing army.

You can heal people by increasing their bodily functions: say the heart -- making it pump several hundred times faster to also kill them.

So, loopholes can be made. But maybe the system is watched over by something or it is half-sentient itself.

It could be that mages require ridiculously high wages, so hiring an entire army is cheaper and gets the same result. Or, magic itself requires some rare metal or liquid or crystal or stone or whatever else happens in the world. Again, context.

Again context-dependent. Maybe the world in question has powerful magic that comes at the cost of a demon pact or hell portal or mages exploding... etc.

This setting is not undone and most of the time magic is still practiced but it would actually give generals pause. You honestly don't want your mage to fall to demons mid-battle and open a portal to hell from which a bunch of demons spawn to destroy your army. If you can control that magic then you throw it at the enemy. But that is a one time use thing. And, again, context.

This is pretty much the story of 95% of weapons programs and how they do before they are born. Weapon X is promising such improvement on the rifle of the army that the army is interested. After a round of testing, weapon X is thrown into the trash because it is unreliable. Soldiers hate anything unreliable and prefer any reduction of anything except reliability.

The praetorian guards literally auctioned the imperial throne of Rome at some point.

Also from Rome: after the Marian reforms the army, usually more than one legion, started to have a very strong bond to the general. Cue the civil wars and we have the armies deciding who is the next emperor by simply backing them up and marching on Rome.

So, the people of your world may have figured out that it is a bad idea to even recruit mages.

You have 10000 soldiers who are fighting. A mages steps out of his tent and rains fire on the enemy; he also heals the wounded soldiers. He inspires them and might even destroy an entire fortified city with a snap.

How long 'til that guy rises to be a warlord?

History tells us that, even without magic, this is always the case and a big risk. With magic it is amplified a billion times. People follow charismatic leaders to death all the time in real life. So, context.

I'm pretty sure I can go on and on. So. Again. Context.