We can't and that's the problem. If we do that welfare charities will run to the European court claiming they're being discriminated against, that their fundamental human rights to have as many children as they WANT is being impeded - even if what they want and what they can AFFORD are two different things. We live in a society today which expects to be given and screams when it isn't, with leaders who capitulate immediately to public and media pressure. Yes they can cut benefits, but the parents with 4+ plus and no jobs? They'll end up on the front page as being made to beg for money by the heartless government.
No politician wants that, which is why it will never, ever happen.