Americans are exempt from the double-tax up to $91,400. Since I imagine most expat workers are earning under this level, I don't imagine that taxation is what's keeping Americans at home. Personally, I've found it difficult to even be considered for work abroad, usually due to visa issues. Other countries seem to have reciprocal working holiday visas for young people, and I imagine that this allows people to develop the specialized skills that justify the visa sponsorship or permanent resident status necessary to continue working abroad.