It’s because they don’t want any kids.
Sure, there are more specific “reasons,” but they are all vorgeschoben (phoney), whether the people giving them actually believe them or not. Like how in the latest survey two-thirds of childless couples asked actually want to have children but their financial situation, say, is too precarious or having kids would make their lives even more stressful than they already are, boo, hoo, blah, blah.
This all makes sense, sort of. As we all know, the human kind has never had children when the financial situation was precarious or the situation was stressful, right? Whatever.
No, they know perfectly well why they don’t want any children. 1) Once you have a kid it’s no longer about ME, MYSELF and I anymore and 2) Once you have a kid you would then actually be expected to take responsibility for it YOURSELF (even in Germany, well, for the most part). And taking responsibility for things just doesn’t cut the Kuchen around the country here these days.
79 Prozent finden den Alltag auch ohne Kinder bereits anstrengend genug,
Observing Hermann 
And then there are the Benefits-class, breeding N children from N+1 fathers, just to get more social-security money 😦
German girls should just take the early lead from the US and get pregnant while still a teenager.
The non-negotiable items are: (i) one to two foreign vacations a year, and (ii) an upscale car of recent vintage. Only if those requisites are satisfied will a child be considered. It’s not the stress; kids crimp one’s style.
And those silly Americans even spend money on education for their children. Education is a right, not a privilege, and someone else should pay for it.
I will continue to enjoy watching parents raising their child(ren) at home, forced to slave away to pay other parents (both working) to take care of their children through forever increasing taxation, so they can afford the outsourcing fee to pay big-governmental strangers to bring up those yucky icky gross and noisy children of theirs.
We also have a low birth rate in Singapore. Neidrig geburtenziffer.