They can't becuase of the following GB directive:
"The most important work we can do in this system of things is the preaching work."
"Their is a need in the domestic field."
"It's all being worked from above."
"Let the will of Jehovah take place."
"Because we are not needed here, it doesn't mean that we are not needed by Jah."
"He may need us in another form of service."