Hey Alex!
It’s partially a matter of degree, but not only that. Let’s dig into it a little deeper.
Trying to hold both the spell array (the Word) and the Will to enforce that spell array in one’s mind, without the ability to split your Will, is obviously quite difficult. Most of the thaumaturgic population never bothers to attempt it, just like most of the current population never bothers to try to get into NASA or compete in the Olympics.
Of those who do try, many find that after years of work they still retain mental blocks that stop them from free-casting. Similar to the mental ruts that we have shown in detaching the output of a spell. Eventually, they give up. These are those you mention who can’t learn at all.
Once you get past those, you may be able to free-cast the simplest of spells. But by that time, you’re likely already an accomplished thaumaturge that has the funds/resources to get potions or artifacts. Those can produce much more powerful and complex effects than what you can do with free-casting, with basically zero effort. They’re small enough to be portable and varied enough to be convenient. Maybe you’re even an artificer or alchemist and can recharge them or make more yourself.
It’s akin to a cross country truck diver using their GPS instead of memorizing the directions to get everywhere or, for people in a mathematics field, using those handy graphing calculators instead of doing the math not just by hand, but in their head. Except that trying to do the math in your head has some chance of killing you, every time.
For most people without Professor Lacer’s talent, free-casting doesn’t just become easy once you’ve figured out the mechanics and gotten past the mental blocks. Each new spell is difficult and it takes quite a lot of proficiency to become “comfortable” as a a general-purpose free-caster.
I imagine that a lot of master or grandmaster level thaumaturges who have kid-level spells under their belt will try for their next spell, almost lose control, maybe get Will-strain, and get frightened off of continuing. All of their effort up until now, their time, and their lives are at stake, for an ability that most people don’t actually need to live happily or to be considered a success.
Finally, a lot of powerful thaumaturges tend to let their fine control slip a bit. Just a bit. Because they’ve got the power to make up for it, it doesn’t matter if they’re 100% efficient. But if you want to be a free-caster, absolute control always matters.
Some baby free-casters continue to learn, and they are around, but they get very much overshadowed by Thaddeus Lacer flaunting his skills everywhere, all the time, like an obnoxious asshole. (I mean, from their POV this guy is too lazy, or too arrogant, to move his own pen. And he goes around free-casting fireballs in the first day of classes, while calling it amateur stuff. So you can’t get much clout among the student body if you show them how you can shoot sparks or make your paperweight float. Especially because actually demonstrating the spell arrays you use is part of how your students learn. Also, is Thaddeus Lacer free-casting a breeze spell on himself to make his coat flap so dramatically whenever he walks? Why doesn’t that asshole just float around on an invisible throne all the time?)
However, several more powerful characters readers are already familiar with do have some minor/limited free-casting ability. Those who have learned anything moderately complex have generally done so within their specific area of interest. I’m not sure if it will ever become relevant to the story, but just in case I won’t mention who they are to avoid spoilers.