For moral, economic, and practical reasons.
From a morality standpoint, your question is like asking why is it an employer's responsibility to ensure that workers can afford food or housing? Why does an employer have any responsibility to pay workers anything?
It all comes down to the value the workers are providing, and morally how that value should be divided. If an employer is making enough money from the effort of his workers to pay not only the employer's own reasonable needs (e.g. other costs, and if the employer is an individual, his own food, shelter, healthcare, etc.) but additionally his workers' healthcare, I think he has a moral duty to provide it, rather than take that money as an additional bonus for himself or his investors.
But even putting aside the more contentious morality question, which I'm sure you disagree with, there is still...
1. Employers will do better if their employees don't get sick often and have to take off work (or worse).
2. Employers will do better if the country as a whole is healthy and less worried about future health-related financial costs.