Why doesn't the western world fix their low birth rates by simply using tax money to pay people to raise kids?
A country's human resource is its most important resource. Increasing the size of your population is the easiest way to increase your nation's power relative to the rest of the world. The best thing you can spend your tax money on is increasing your population size; there is no other investment with a higher return on investment.
For example, in order to quadruple the USA's population to ~1.2 billion people in the next 50 years, it would only cost taxpayers ~$222 Billion dollars per year to raise those kids. For reference, that is only ~6% of the USA federal tax budget of ~$3.5 Billion per year.
Or, you could continue to do nothing, save that ~6% on your taxes, and watch your country get taken over by the rest of the world.