Because most countries aren't actually trying anything nor can the government do much in that regard when the issues are cultural and emotional.

The low-birthrate couples who can't/don't want kids aren't suddenly gonna want them if you give them 3 months of paid maternity leave and a monthly stipend or other such welfare perks.

And those who do have kids because of government welfare perks, were having kids anyway, they're now incentivized to have even more.