Then those who prioritize it will take jobs where it is available.
By mandating it, an employer has to consider the possibility whenever they hire someone, and so salaries will be lower as a result. If the employee doesn't get pregnant and use the leave, their salary has been kept lower with no benefit to them.
The expected benefit for those that don't use the leave themselves is a more stable and healthy society due to better child care and stronger social/family bonds. Overall lower (net) salaries is a trade-off that most societies in the world are willing to make. Mandating it is a means of mitigating the tragedy of the commons - people thinking that there is no benefit to them at all when they don't get the benefit directly.
By mandating it, an employer has to consider the possibility whenever they hire someone, and so salaries will be lower as a result. If the employee doesn't get pregnant and use the leave, their salary has been kept lower with no benefit to them.