you only have to look to China or Korea for the answer to that.
30 years ago, a 30 year old would have parents of around ~55 who may soon need some support, and perhaps one grandparent of ~80 who they would have to consider providing for too. having two kids was perhaps economically viable, but disallowed.
in 2023, that same 30 year old has parents nearing retirement, likely two grandparents who need financial support (which will fall to their highly educated grandchildren to provide, seeing as their children were educated and entered work pre-economic super-boom period) and have plenty of years left to live, the average lifespan having grown by +10 years since 1990.
so that 30 yo is thinking: if I have two kids, or even one, in 5 years there'll be four generations of my family alive and I'll be the only earner. and of course people are not gonna want to have kids knowing that two generations above them and one below will be partially dependant, maybe entirely dependant, on them.
as for what can be done to actively incentivise people to start having more kids,
the answer is not much. most efforts are a complete flop, and even in places like Sweden all they've managed to do is pause the decline in birthrate, not actually reverse it. global population
will fall significantly in the next 50 years, which as
@VoR says may not actually be the worst thing. but an ageing population is never a
good thing as it puts so much pressure on an increasingly smaller pool of people to provide economically.