Many do, but the problem is that unlike new tires, which are a one way trip, retreading involves collecting, retreading, then returning the tires to the same location. That means the retreading site needs to be fairly local...... But I thought the tire makers had their own retread line as well?
What is pretty common is for a trucking business to contract with a tire dealer to supply both new tires and to handle retreading - and sometimes the guy on site handling the tires is an employee of the tire dealer. It is not uncommon for a name brand tire dealer to use a different retreader just because of the convenience.