Man people self-report a lot of fun stuff. It simply cannot be trusted. The counterpoint is that nations that tried financial incentives (even large ones like Hungary) had negligible gain from these efforts.
Let's look at Hungary: loans unless you have 3 (!) children, maybe some housing subsidies that require you to match them with a huge chunk of your own money. Small wonder why that didn't motivate many people.
What countries do want is to incentivize production of properly raised, well mannered, productive children. And the type of person incentivized by a lot of cash to have a kid is probably not going to be the one to do that.
I think the bigger change is: I don’t want my quality of life to go down one bit when I have a kid!