system
Given the following post, respond as if you're commenting on Reddit:
human
[The Wheel of Time] why don't the aes sedai have sisters posted in the borderlands to help guard against shadow spawn?

The books talk a great deal about the borderlanders job of guarding the rest of the world against the invading shadow along the blight.  The people there have a deep reverence for Aes Sedai, unlike many of the other countries so they'd certainly be welcome.
The Green Ajah is "the battle Ajah".  One of the three oaths is to only use the power as a weapon against the shadow.  This all implies part of their focus is to fight the dark one.

Despite all this I don't understand why there wouldn't be a regular group of sisters posted up there to help out with the fighting.  I know their over all numbers in the Tower had been falling but still.  Greens at least, and you'd think yellows as well to help heal the men.    At best there's mention of a sister as an advisor to the kings or queens but never a garrison.  Iirc correctly when the Shienarans rode to Tarwins Gap way back in book 1 they had no Aes Sedai support.  The books definitely imply that the men up there see action regularly, if I was them I'd be pretty frustrated that the most powerful women in the world aren't lending a hand on the regular.
gpt
Alas, the various Ajahs of the Aes Sedai pretty much no longer live up to the standards they originally set for themselves. Even the Forsaken mock them for it— not just for their lost knowledge and decreasing power, but for their lack of vision and action. 

The yellow ajah are supposed to be healers, but they don’t go out into the world using their healing weave, they tend to stay locked up in their wing trying to practice and refine the one healing weave they know. Always trying to improve themselves on it, rarely ever actually USING it. 

The green ajah are supposed to be warriors, but instead they collect men like girls collect Barbie dolls, and act more like a high school clique than anything else. 

And so on. Of course, there are ajahs that do ostensibly follow their stated goals— but not that well. Whites study all the time, but they rarely learn anything new. Blues meddle in politics all the time, but rarely accomplish their goals in doing so. And reds... well, yes, reds gentle men who touch the power, but they’ve been so infiltrated by darkfriends that they may as well be entirely black. 

So that’s why. Yes, greens and yellows SHOULD be on the borders. But they’re not, because they don’t live up to their ideals.