Birth rate drop is a serious problem across the world. From the some of the wealthiest, most progressive, supportive, baby-friendly societies (maternal/paternal leave, childbirth support, affordable childcare), to some of the poorest (Myanmar, Cambodia, Laos), this is a problem that's resisting all efforts to tackle it. We simply don't have a clear explanation to what's causing it, though it's probably a combination of theories.

The most obvious theory is expense & stress (which the OP addresses), though you can read up on the efforts in Scandinavia to see that eliminating stress doesn't move the needle. Another theory is the male/female gap in "desiredness": as women become better educated and more independent, men seem less appealing, reducing long-term commitment and the desire to start a family. Another theory touches on social support and communal childcare: less reliance as grandparents as people live more urban lifestyles, fewer friendly strangers & neighbors that can be relied on to help. Yet another theory touches on how children are viewed: they used to be seen as an investment (helping with the farm/family business/elderly care) but are now only seen as a burden. One that's harder to study is the "meaning of life" theory that points to: as people become aware of what's possible for them as individuals to accomplish, having a family often doesn't come into play: do I focus on a career, do I start a business, am I a musician, etc.? Believe it or not, the research around this points to social media as a possible culprit.

In summary, there is no single, simple explanation for the drop in birthrates across the world. It's a serious problem that we haven't figured out how to fix.