Unique Spell Components ###
Every spellcaster's magic is just as unique as they are and each one needs slightly different spell components (brown bat guano versus free-tail bat guano for fireball) or to have those components stored/prepared/handled in slightly different ways (this mage needs his silk balled up while this mage needs his silk strands combed straight). Imagine the logistical challenges that would face an army supplying spell components to 10,000 wizards each of whom needed their own unique variant of the component.
We do the same in armies in our own world. Interchangeable weapon parts are hugely important and the US navy is willing to accept the limitations imposed by a standardized launch tube to get the benefits of missile compatibility.
Component Depletion/Access
Maybe it's relatively easy to get spell components in the relatively modest numbers that a few adventurers need for the occasional brawl and some studious academic wizards need for research but trying to procure them at scale becomes virtually impossible. Spells might require components be picked naturally in the wild and not farmed/planted. 10,000 casters casting spells as fast as they can might quickly denude a kingdom of components in a few days and as they become harder to find you need more men out looking for them.
Worse, what if each caster must gather their own components themselves. That might be trivial for a small party near nature but not at the front lines. You can't constantly have your mages tromping back and forth to the front to go pick their own herbs or pieces of dung or what have you.
Variation and Coordination
Magic users are each unique making them virtually impossible to train or deploy at scale. Maybe what determines what spells/effects each caster can accomplish isn't as much a conscious choice but deep facts about who they are so you can't train up mages with a common arsenal of spells. Imagine the chaos unleashed if a 100 casters each cast totally different spell types at the same time. The Fire elemental one caster summons bumps into the Water elemental another summons. 227
One caster tries to blind the opponents with blinding light while another uses magical fog/darkness.
Maybe gods in your world do use armies of spellcasters but they are the only ones who are able to remember and utilize 10,000 different individual skills effectively. Mere human strategists need an abstraction like a unit with known weaponry and capabilities to effectively plan and coordinate battles.
Training Is Unscalable
Maybe magic isn't the sort of thing you can instruct people in (or have them practice at) in a regimented highly scalable fashion. Each individual wizard needs quiet contemplation with only a few colleagues to learn their arts or to improve them. This makes it impossible to keep them together in large numbers in barracks (why they always going off to isolated towers) so you've got to go gather them up before you can use them in battle and better hope your enemy doesn't show up in the meantime.
Not to mention all that alone time makes it difficult to ever find time to train them as a unit.
Personality Unsuitability ####
Also, spell casting might only attract a certain type of person of require that type of person to succeed at it. Consider the difficulties modern armies have recruiting and keeping computer hackers or scientists. Mass combat requires quick, unquestioning obedience and conformity while wizards (like hackers) are only any good because they question and respond in unusual ways. Or perhaps most wizards are very absent minded, or quarrelsome or have some other issue that keeps them from being good soldiers.
Fatality Rate ###
Maybe it's particularly easy to target other magic users on a battlefield with certain spells or tactics and very hard to defend against. Thus, in each battle the magic users (not the grunts) wipe each other out pretty quickly taking decades of training with them as they go. If mages die in wars at some crazy high rate you'll find it virtually impossible to get volunteers and a bitter magic user who is compelled to be there is more trouble than they are worth (or they are simply too powerful to capture).
Maybe the magic user union stepped in and laid down the law to prevent drafting magic users against their will on pain of severe retaliation.
Spacetime Magic Limits ###
Too much magic used in a small region produces catastrophic uncontrollable effects (kingdoms get turned into craters, gateways to horrific planes open releasing horrible fiends. Or maybe too much magic in one place is like hearing too loud of a sound for magicians and it (permanently or temporarily) removes their magical ability if they are too near too much simultaneous unharmonized magic.
Cultural Norms
Using magic is like targeting officers back in the Napoleonic wars. It's considered dishonorable and anyone who does it gets blackballed in politics and can't find any allies. 227