Comment by advisedwang

The reason why is the drafters of immigration law care a lot about US employers (as they are voters, donors and have a big lobby). US employers want to control the green card process so that they have leverage over their employees. If employees can get green cards without the company, they can leave or threaten to leave. If employees need their employer, they can't start the processes until the company assents, and can't leave while its in process. Depending on exact details that can give employers 5 or so more years where the employee is at their mercy.