Why can't body hair be transplanted to the eyebrows?

Body hair can be transplanted to the eyebrows.  Some may use arm hair, leg hair, armpit hair, chest chair, beard hair, etc.  In general most doctors use scalp hair because it gives consistent results.  Body hair is not as predictable in terms of the success and growth rate because the hair cycle is very different.

This is certainly possible. However finding a surgeon who has a lot of experience doing that type of procedure will be difficult, since most cases it makes more sense to take the hair from the back of the head since the scar will no be visible there, whereas if hair is taken from the arm scarring will be visible. I suggest doctors who work with eyebrows often, as the source of the donor hair is likely less important than knowing how to transplant to an eyebrow when achieving the final look.

It is possible and can be done, technically speaking. It is just preferred to use the donor hair from the back of the scalp for many reason. See the article below for uses of body hair in hair transplants.