Children are not a lifestyle choice in the same way as deciding to extend yourself to afford a bigger mortgage, choosing a holiday instead of a week in the UK, etc. Children are the way we perpetuate the species and we are biologically programmed to desire this. Being intelligent creatures we are able to apply thought to this and choose when and how many, but for most people, the urge to have at least one child is as much an instinct as eating. I think that's often forgotten by the "don't have children unless you can afford them" brigade.
Upwards of 80% of people in Western countries (higher elsewhere in the world) have children. It is a fundamental urge ensuring survival of the species.
So, given that we're all going to keep having children, and unless you subscribe to the idea that only the rich make good parents, we are going to have a situation where society has a lot of children who need looking after, and in many cases it is unaffordable for that to be done by a paid-for child-care professional.
We could say tough shit, let's not provide any help, but that would leave only the rich having children. 
We could make childcare more affordable. Good move. 
We could recognise that since a child needs caring for perhaps one of its own parents could do it.
Why on earth can't that be recognised as something that is helping society rather than just the family. A tax break or two for that really wouldn't be that much a hardship, surely.
BTW am a full-time working single mother who has always used professional childcare.