They couldn't "back then" either, I would argue.

"back then" a family could live on a single income.

This was largely for relatively privileged people. Most families the mother worked as a teacher, nurse, domestic, secretary, nanny, etc.

Common misconception. Poverty levels were far higher. In 1950 half the global population lived in abject poverty. Today, it's less than 10%.

https://ourworldindata.org/poverty

They had to. Outside a few vocations, women weren’t allowed to work.

And now they have to work because a single income is no longer enough. Yay for progress.

Some people could. There was also a lot more abject poverty.

Working a minimum wage could buy a starter home "back then". It now can hardly pay rent, and starter homes essentially no longer exist, even if someone wanted one.

It's not just that young people can't afford to have babies is that women in the 20-29 range are either studying or entering the work force.

These are extremely important years for career building.

It's more about the fact that young women are choose to build their education and career (like men do) rather than have children.