Non-capital recruitment is something that even the dev team has discussed from time to time. Our best idea was to tie it to duchies. Make it so that in the duchy's city (this was back before the new allegiance system, and back then city=duchy) was a place where nobles of that duchy could recruit troops trained in regions that belonged to that duchy. The problem with this is that anyone who doesn't like this restriction can completely avoid it by swapping regions into the duchy closest to the enemy. (That's even easier now that duchies don't need to be contiguous, you could just move around regions with high-demand RCs.) Any noble who's lord doesn't want to swap duchies can just grab an estate in a region of the strategically-placed duchy. Gold is no object, since they can be given as much gold as they need by anyone in the realm who has it. So you have a mechanic that only limits people who allow themselves to be limited.
The usual response to this is to say something like "No they won't!" Or "Not everyone will do that." Or "Put in restrictions on duchy size, or a duchy efficiency, or a distance-from-duchy-capital penalty, etc." i.e. in order to make this system work we have to make it more complex by adding new mechanics to address the loopholes it opens. Like, for instance, limits on how often you can swap duchies, or severe morale/loyalty/control penalties for swapping duchies. Or we can add arbitrary OOC restrictions on recruitment, such as only crappy troops nobody would want anyway, or only 10 troops/RC/day. i.e. add enough limitations that the non-capital recruiting is nerfed so bad it's useless. And then we have people complaining about the borked non-capital recruitment, and calling the system broken and worthless, which it would be, and why the hell did we bother spending dev time to implement such an f'ed up system anyway?
And all these the extra mechanics make the system more and more complex, less transparent, and more bug-prone. Or we just don't add them, and implement a system where the only people that are limited are those who voluntarily submit to the limits, and even then this is unlikely to limit everyone.
And if we try to regulate it by rules, then we open up other possible abuses like "strategic duchy swaps/creations" or even "strategic oath abuses" or other such nonsense.