Lack of trust. Being evil. Adventurers are weaker than they need. No adventurers around. Type A personality. Don't like adventurers. Must be harvested yourself. There are literally thousands of reasons why they wouldn't hire adventurers.
And a thousand reasons why they would as well.
"Mages need magic items" is a weak reason why they would go to "random mountain village #3" more than once every 60 years or something. But if you want to insist that all wizards are travelers who visit every minor part of the realm just to prove a point that they should be seen in villages where the height of comfort is the bed with the fewest lice in it, I guess I can't stop you.
When was the last time you saw the law transform a person into a frog? Or the last time it uprooted a mountain in 6 seconds? Or granted wishes? Or teleported someone hundreds of miles in an instant? Knowing the law like a lawyer gives some power, but not nearly what magic does.
A line drawn on a map can cause hundreds to die, a single law passed can take that mountain and turn it into billions of gold coins.
And really, don't get too caught up on the upper echelons of magic. That is a lifetimes work to get there, a decade of studying law can make you the most powerful person for miles, with the backing of armies if some towered hermit decided to start threatening you with toad life for a measly hour.
All that means is that not everyone would pass it on, but many would, and many of that increased number would as well. Eberron is the logical result of a world where everyone can learn magic.
That just results in everyone eventually being a wizard. Wizard teaches his 2 kids. One kid grows up and teaches his 5 kids. The other doesn't marry or have kids. The 5 grandchildren average 2 kids each, so now we have 10 more wizards all teaching their kids. And it keeps growing.
Populations grow for a reason.
It doesn't grant wishes. It doesn't cause great balls of fire! It just plain isn't as desirable as magic is.
C'mon Max, you know this isn't how things work.
Wizard teaches his two kids, they both learn magic. One doesn't have kids, the other has five. And supposedly this one wizard taught all five of his kids and they all became mages.
None of them went into other work, none of them just didn't have the talent for it, none of them became artists, all of them get married and have two kids each even though the first generation had a 50% rate of no kids.
And everyone survives into adulthood, no one seems to die on all those "Realm Hikes" to the most obscure parts of the world you want to send them on, or have lab accidents. None of them get in debt and end up struggling to make ends meet.
Life happens to people Max. And just because you see the ability to catch someone on fire as being something no sane person would turn down doesn't mean that is how life will work. Heck, I work as a substitute teacher, I can easily point to the fact that math and reading can save you millions of dollars from unscrupulous sales people. I can point to history being some of the most fascinating stories about anything you could ever want to hear about. And I can also point to the fact that kids hate work, and they will chose not to work even if it actively harms themselves in the near future.
Life isn't a math problem that ends with everyone knowing the same skill.