Because low-skill isn't unskilled. Employers know that even in these jobs a modicum of experience gives an applicant a decided advantage in applying for new job

Because low-skill isn't unskilled. Employers know that even in these jobs a modicum of experience gives an applicant a decided advantage in applying for new jobs, and not having it serves as a bar for employment which means that this underpaid labor also has to beat the cost of retraining, then likely accept a below-market training wage to get work. At the same time these clauses reduce the number of potential employers for any given candidate, so any given employer has less incentive to meet a higher salary demand.