Because it would be a nicer world to live in (in my opinion and many others') if becoming a parent didn't practically require sacrificing one's career and thus future livelihood. Mind you, I'd be happy to see paternity leave paid from a general tax base rather than necessarily from a particular employer, but the general principle that sometimes entities should pay for things they might otherwise not want to pay for to help make society more pleasant to live in is just the same as for myriad other generally accepted expenditures.