Im guessing its because of visa standards and requirements. In the situation described above, the employee would have to get a visit visa every week. Weekly costs, including air travel and hotel lodging and boarding would top 2000-3000 dirhams…adding up to 8000-12000 a month in such expenses alone. And then, having someone work while on a visit visa, which in itself is a violation of immigration law.
If on the other hand, the person is sponsored for an employment visa, he would still be cheaper if retained in dubai (ticket and lodging charges easily add up to 10,000 a month.
But Im sure there are people who probably do that. I know some businessmen who do exactly that.