1) Shifting Culture of Norms == Children are expensive. If you have kids you might have to give up the lucrative career opportunities you have been building towards since your university days, and other personal opportunities
2) Inflation of expectations on parenting == Children are expensive. Parents are expected to spend a lot more of their free time and money putting their kids in programs and sports and such
3) Delay in child bearing led to fertility drop when people really want to have kids == Children are expensive. If you want to have them, you need to have them before you are really financially stable and established in a career.
4) kids are no longer useful == Children are expensive. In the past kids were expected to contribute to the household at a fairly young age. Nowadays not so much. This means families either hire someone to do that work or it falls on the parents.
5) Women in the workforce == Children are expensive! Families have an entire extra income with Moms working and they still can't afford childcare. With an entire extra income!
So you're right that the reasons for falling birth rates don't have to be "children are expensive" but all of the reasons are certainly "children are expensive"-shaped