WHO's responsibility is not to a single person. Their policies are designed for the society as a whole. Minimizing the possibility of infection for critical jobs is much more valuable than minimizing the infection probability to a single person. Society is more than just me or you or our families.

They could have taken the path you are suggesting (possibly under inconclusive benefits of other types of masks) but that could have backfired and caused a lot more dmg than good.

You can't expect an entity like WHO to solely care about you as a person.