Because it was one of those conditions hard won with blood, sweat and tears by our great grandfather’s generation in order to ensure people are not penalised for being sick. Or children for being sick and requiring care by a parent.
Plus a single day - which is usually all most people need to get over the worst of the vast majority of run of the mill sicknesses - of wages is generally a lot less of a loss than the potential loss of a large section of a work area’s labor if someone who is sick comes to work and infects their colleagues. And this is not an open lather either; everywhere I have ever worked will allow only a single day in a week without a proof from a doctor or other medical professional that you are indeed sick. And if that sick day was a Monday or Friday, you needed the cert regardless.
Another way to look at it is a benefit in place of raw cash: paid days off for illness recovery, care of children or relatives who are ill, in exchange for a reduction in overall wage.