There are plenty of qualified technical professionals who could teach math or science if the certification hoops weren't so hard to jump through. I have a friend who was an independent contract programmer, very literate and well-versed in the history of science. Work in his specialty area was hard to find, so he explored teaching. He was told, yes, technically you're qualified, but the district can hire a certified, fresh out of school teacher who knows a fraction of what you know for a fraction of the price.
Foreign teachers must be brought in from other countries because qualified U.S. teachers WILL NOT work in the rotted inner city school districts the Democrats have run for decades.