Many reasons: Polishing for chrome plating is dirty, tedious and dangerous work . Efficient polishing requires horsepower. Most platers use wheels with 12 for 25 horse power motors, running at 1260 RPM. A mistake that causes the buffing wheel to catch between the stays and seat tube can result in severe injury.
Then there are the hazards of exposure to strong acids, heavy metals, dusts and high voltage. Platers earn hazardous duty pay! Meanwhile, most quality plating shops have a clientele of motorheads who throw thousands of dollars at the chrome on their projects, which rarely present the hazards of our bike frames! Lastly due to the hazardous nature of plating it is very expensive and difficult to get the permits to build and operate one.