I think the map isn't very helpful. I would be interested in seeing how many women have access to maternal leave in the U.S. I best the number is surprisingly high. Also, many small employers are willing to work with parents. A mandate doesn't give any flexibility. For example, perhaps a parent would prefer going part-time for several months instead of 6 weeks paid. Employers compete for hires, why not allow them the flexibility to design their own benefits?
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.