Children cost hundred's of thousands of dollars to raise, and that doesn't even count the opportunity cost of your own career progression as you have to spend a year of sleepless nights and possibly have one or both parents reduce their work hours to care for the child.

The Japanese government already struggles to pay out pensions with its aging population, healthcare and pension costs are both rising, where do you propose the money for this comes from?

Should the government increase its already high tax rate from up-to-50% to up-to-90% and take money from the childless to give to parents?

Should the government replace your salary if you quit your job to raise a kid (since after-all that is a cost of the endeavor)?

If you're just talking about "giving birth", I assure you the cost to give birth is already close to free, the government already covers that, and various cities have local parent stipends which make it "profitable" in a sense.

But the real cost of giving birth is not the giving birth, it's the millions of yen you then need to spend over the next 18 years to raise and educate the child, not to mention the cost of possibly dying during childbirth.