Mostly just economics, is my wager. In a classic agrarian economy, having a spouse and making children works out -- more children = more hands = more work done = more food or more product to market. In a modern "pure" capitalist industrial/service economy, the economic model doesn't pay for the cost of the reproduction of the labour it depends on. So not a surprise when people do the same cost-benefit analysis for th…
It's all doable, of course, but all risk has increased. One thing I would do in my life on a do over is start earlier.
Late 30s you're playing on hard mode. Early 40s, you've got to sperm donor that and solo it (which a friend of a friend is doing). The time risk is very high!