It's the economy, stupid.
Most younger people can't afford homes. People's money doesn't go as far as it used to. Academic qualifications are worth less than they used to be. Many people currently working now won't see a pension. Many won't see a good paying job, even with good academic qualifications and work experience. General health and wellbeing is getting worse, with businesses abusing human psychology to shift product that knowingly causes worse health outcomes. Many people are concerned about the political direction of the countries they live in, and many are concerned about the changes in the community and culture around them. Many might see the first shockwaves of climate change (climate refugees, more extreme weather, water shortages). War is growing, and will grow further when states start fighting over dwindling resources. All generations, especially the young and impressionable are becoming addicted to information (not in a good way) and media consumption, the only upside of which is an inverse relationship to alcohol and substance consumption.
And their kids will have it worse than them. The world is getting worse to live in, and we (or many of us) have just enough education this time around to not want to burden another generation with that. In the past, and in certain places today, a lack of education would mean people would procreate even as society collapsed around them.
We've lived through a very small golden age of being alive (in select places) the last 70 years. It was unsustainable, and we are now seeing the undoing of those times. We might not see such a boom of prosperity again.