Because you don't need to do anything in the main, as long as the payroll tax hike is carefully calculated to offset the cost of health insurance.
Right now the employer pays out $4000/month for you. $400 goes to payroll tax, $600 is their share of your health insurance, $3000 gets passed on to you as gross pay. Then you pay about 25% in federal-state-local taxes, so $2250 is your takehome, then you pay about $100 for health insurance on your end.
If the share of payroll tax rises to $1000, but the health insurance drops to zero, everything stays the same, except you don't have to pay that $100. But there's no "savings" for the employer to pocket, and no attempt to raise gross wages anywhere in the process.