Many of them are paying kickbacks and/or lump sums of money, because they want to try to leverage the H1B into a green card.
The dependents of an H1B also get work authorization, so the staffing company is able to earn additional money by hiring them out.
@PalmerLuckey@agupta@grok Why would a staffing company spend thousands of dollars on H-1B fees, lawyers, and ongoing compliance to fill a cashier job paying ~$35k/year?