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 themselves and choose (or are forced) to have no children, and to not bother with marriage as a result.
The no-children "choice" often also ends up with no partner -- married or not. I personally knew several women in their late 30s and 40s who ended up leaving partners or leaving the dating scene because the men they were finding were not interested in or capable of being fathers. On an aggregate scale this adds up to a phenomenon.
Some advanced capitalist nations have gotten around this through high immigration rates. Others are attempting to get around it with debt to finance social programs or stimulus etc. In both cases it's really just "shooting the puck down the ice" so to speak. The next generation or the one after has to deal with the problem.
- The share of adult children who live with their parents has been increasing -- unsurprising given the cost of housing. But while a millennial might grudgingly get their boomer parents to put up with extended cohabitation, that situation makes it hard to find a mate, and asking a partner to move into your basement bedroom is probably a non-starter.
- Especially prior to the pandemic, there has been a long trend of workers were becoming less mobile. Likely both partners in a marriage need to work, but housing that works for one partners job may not work for the other's.
- The share of workers with multiple jobs (or one 'job' plus gig work) has increased. As you spend more hours working just to stay afloat, of course it's harder to find and nurture a relationship.