I very much question the idea that its harder to start a family these days than it was, say, a hundred years ago. But even assuming that was true, why then is fertility rate inversely proportional to income in developed countries? Surely the wealthy who have more ability to start a family should be the ones having more kids, then?
Because of women in the professional workplace.
Essentially every society on the planet going back to the dawn of time aculturates women to prefer a higher status male as their mate. Status being defined generally by wealth, education, and profession.
As women have worked their way up the economic ladder they have made their own status higher while simultaneously reducing the number of high status jobs that are available for men to hold. This has the effect of reducing the relative number of acceptable mates and consequentially you have more single women and thus fewer kids.
The reduction in social consequences from being an unmarried women and having premarital sex have also reduced the incentives for women to marry and have kids.
Then you have the lengthening of childhood. A hundred years ago, leaving home at 16 was nothing special and if you weren't working by 18 then you were basically a bum. You were expected to be married and setting up your own home by 22 or so. Today? We barely count people as adults getting their first job at 22.
We are pushing the start of adulthood (i.e. living on your own, getting married, having kids) much closer to 30 than to 20.
Kids are also, as a percentage of income, much more expensive today than they were a hundred years ago. They are also in many respects harder to raise today. A hundred years ago virtually every mother was a stay at home mother and had their own kids; they also all went to the same churches and social clubs and grandparents usually lived in the same town. Communal child rearing was a lot more of a real thing than it is in most places today.