1) You can hire temp workers on visas relatively easily and in large numbers. This benefits employers, politicians, economy, etc.
2) These workers need permanent status. US immigration is uniquely regressive in that everything flows through the employer in most cases and visa holders have no agency. So the employer needs to petition for a "permanent" employee.
3) So although these people are already employed, the employer needs to go through a PERM process where they advertise the job and claim to the gov. that they couldn't hire anyone else and this is the most qualified employee for the job.
4) Although it's a farce, DOJ gets mad if there is a perception that the employer tried to make the PERM process more favorable to the employee they already have.