Dumbasses, it's social media. Social media makes it impossible for people to get along, which is why young people "struggle to form relationships." Social Media trains people to live in a state of defiance and opposition to those around them.
Social media may suck, but this issue predates social media and I seriously doubt it was exacerbated by it. It's always about money. Our fertility rate was high when people could afford homes to raise kids in. Our fertility rate was high when it was easy enough to provide for a family on one income. Those are the 2 factors that are killing fertility rate in the advanced world.
1. Education. College is now mandatory for most to enter the middle class and many occupations expect advanced degrees. If you don't get your first real job until you're 26...yeah, that drastically reduces your chance of having kids.
2. Costs. OK, now you need to be REALLY FUCKING WEALTHY to buy your first home or merely to live alone in a place big enough to start a family. This goes back to issue #1. OK...I can't afford a decent home with a HS diploma, so I get a BA...which again...I can now afford my own apt, but it's too small and cramped for me to start a family...so I go get an advanced degree so I can give my kids the life I had growing up...so...yeah....now you're starting families in your 30s and not your 20s. Additionally, long ago, you could raise a family with one income. Now...you really can't, so the mom has to work...if the mom can devote all her attention to it, the family will likely have more kids. If she's either juggling work and kids or taking off and watching the saving drain...yeah, they're less likely to have a large family.
And it's just common sense...the longer you wait to start, the greater the chance of divorce. LOTS of couples get divorced between age 32 and 38...that's when the person who was wonderful at age 22 either fails to grow into the 32yo you need them to be or becomes a total piece of shit...or you're the total POS and your partner outgrows you...
Additionally, it's a lot harder for most to start families in your 30s than 20s. While I come from trashy stock and there's no such thing as a planned pregnancy in my extended family...many in my cohort needed medical help to conceive. Miscarriages are more common with paternal age as are birth defects. Few have a kid with Down's Syndrome or a nearly fatal heart defect and say "let's have 3 more!"
Fewer people have kids due to our economic and educational pressures.
Those that do?...they start later and thus have fewer...
It's been well studied. You can't really incentivize couples to start families. The most effective route is to fund people who start their families to encourage them to have more kids. If a couple is happy childless, there's no sum the gov can afford to make them want kids.
However, for people like me, who have 2 kids, but can't afford a 3rd...well...we might have considered it if we could get cheaper childcare and afford a bigger house.