The most consistent predictor of the decline of the birth rate in a society is increased levels of wealth, which indicates it's in no way that simple.
Personally, I think it is that expectations rise, both for your own life and for that of children once your purchasing power increases. It's harder to give it up than to not have it in the first place. Having children increasingly has to compete with a lot of other rewarding things that require far less commitment and far lower costs.
Having children has also become higher risk, both in absolute costs - as well as risks associated to the likely possibility of divorce and single-parenthood.
It had plenty of risks before too. Doing genealogy, e.g. of one of my great-great grandfather's children, several died young and left behind children, some ran away from responsibilities (one child - my grandmothers first cousin - ended up in care and was adopted because her mother was too poor to care for her and her dad ran off to the US with his brothers, and we didn't learn she existed until a few years after she'd died) or were forced to work abroad for years at a time. Of 12 or so of his children, I think only 4 were in their children's life until they reached adulthood, and at least one of them was a single parent because her husband died young.
The same great-great grandfathers second wife likewise "lost" a brother that was almost certainly taken into care while she and her older brother were left to fend for themselves as teenagers after being orphaned when their father died after having cared for them alone for a few years after their mother died.
My grandmother's mother - married to one of the children of the same great-great grandparents - lost contact with her siblings in her early teens when they were all taken into care when her dad proved unable to deal following her mother's death. While she found a couple of her sisters again later, her dads failure to cope means even now we've not been able to figure out what happened to the rest of her siblings.
While divorce is more common, abandonment, be it intentional due to the difficulty of obtaining a divorce, or unintentional (poverty, disease, death), or absence for noble reasons (e.g. going off to earn money to support the family) still left a lot of people raising children on their own in often awful conditions.
There's a marked increase in the number of situations like that for each generation further back in my family tree among the people I can track.
I'm going to go out on a limb and say something that I "feel" is plausibly true, but I don't have any data to back it up. While life has been objectively harder in the recent past (prior 80 years), life is subjectively difficult today in a way that it wasn't for the past 200 years.
Folks often start out heavily in debt, then they must work competitive jobs which grow more competitive over time to tread water. The combination of sleep debt, social isolation, and general anxiety of going bust is a subjective difficulty which belies the relative comfort of modern life.
This is born out at least partially in the data where "occupational burnout" affects some 65% of employees in the last year.
Maybe for some, but most people don't get college degrees, for example. Most people worked heavily competitive jobs from an earlier age before, and often far harsher work.
E.g. my grandmothers half brother worked in New York Harbour from he was 9. Most of her uncles and aunts worked from early teens, and often moved from home at that age because their parents couldn't afford to keep providing for them and needed them to get employment. Often in backbreaking work that ruined their bodies, and in some cases killed them young.
People might well have a subjective experience that they have it tough, but if so we're failing in teaching people history.
Personally, I think it is that expectations rise, both for your own life and for that of children once your purchasing power increases. It's harder to give it up than to not have it in the first place. Having children increasingly has to compete with a lot of other rewarding things that require far less commitment and far lower costs.