system
Given the following post, respond as if you're commenting on Reddit:
human
[Wheel of Time] Why do healers need to touch you when they heal you? And why can't they regrow limbs?
gpt
Nynaeve speculates if she can heal Rand's lost hand, but Rand shrugs her off since he "doesn't have the time".  

As to why, in WOT magic is extremely specific.  Weaves accomplish only the effect they accomplish.  There's numerous times that effects that seem similar require drastically different weaves.  

In the case of Healing, this uses Air, Water, and Spirit.  It restores the person's body, at the cost of their energy reserves.  It basically is accelerated natural healing, healing at hundreds or thousands of times the rate you'd naturally heal.  This drains the body, and leaves you hungry and weakened - and doing too much too fast will kill you.

The Asha'men "discovered" a method using Fire, Earth, and Spirit - basically burning and restoring the body.  This heals using pure magic, at a much higher cost to the caster, and is physically painful for the healed person, but it doesn't drain the body's energy reserves.  On the other hand, it's very crude - a bricks and mortar healing.

Nynaeve and Flinn conjoined the two methods, creating a method that essentially drew from magical energy to cause rapid regeneration, allowing complex and intricate wounds to be healed flawlessly (the advantage of the Air/Water method) rapidly and using magic instead of the body's energy reserve (the advantage of the earth/fire method).  These weaves are astonishingly complex, but you'll note all that's happening here is assisted regeneration.

The weaves for creating a new body part that can replace one that is lost is probably closer to the weaves to create new life.  Aginor is probably the only person who understood enough of the art of Healing to accomplish this (and had enough raw power - he was second only to Ishmael), but Aginor is first a Forsaken, and second fucking nuts even by the standards of sanity among the Forsaken.