Employment taxes, including employer portion of Social Security & Medicare (equal portions from employee + employer), unemployment insurance premiums, Worker's Compensation insurance premiums, disability insurance premiums; paid time off such as vacations, sick leave, personal leave; health insurance premiums; any retirement plan or pension plan or contributions to 401k, etc.
Then there are facilities for employees: facilities rent for parking lot space; any coffee/lunch room space & expenses; cost of your desk, chair, computer, etc. (which they don't pay when employees work remotely); any other employee benefits or perks (even the department's monthly pizza lunch), subsidies for bus passes.
Expenses vary depending on the employer, but many of these costs are required by the govt. And the costs listed here are NOT all of the possible costs to employers. Whatever your salary is, additional costs of various taxes and the most common benefits (sick leave, vacation) add at least 35% to costs, and often exceed 50% of salary.