Probably not to do with liberalism as illiberal societies are also suffering from the same phenomenon. The issue can be isolated to economic development, which itself is a result of female education and subsequent participation in the formal labour market. Turns out being a wage earner / consumer is in tension with being a parent, especially a mother. Interestingly, remote work seems to have an effect of increasing birth rate, we might be able to solve this by redesigning work around family making, rather than treating workers as atomic individuals with no social relations outside of the market