That's not entirely true. Some YA books I have read have legal adults as main characters. The thing is, they tend to be in the military.
The Adult Problem does not exist when the problems the main character faces are age-appropriate. A teen can have to learn not to abandon interests in the name of popularity, or trust flattery, or what have you. Becoming an adult opens up more venues. Andre Norton's Catseye opens with the main character going to a labor exchange looking for a temporary job. That he is an orphan makes it more urgent in some respects -- less in others, he at least doesn't have to worry that his parents can't find work and he may be the only paycheck. But that's because he's a displaced person not in a formal structure such as the military.
Now, an adult in the military can reasonably tackle more problems than a minor. But a newly enlisted soldier, or a very junior office, has to have reason to be the person making the decisions that the plot requires, most of which are often far above their grade. All their superiors have to be absent for some reason, or evil and so part of the problem, or incapable for one reason or another. (And incapacity for incompetence is even more interesting in the military than when minors have to wiggle around adults.)