In the 1920s there was no social security so having kids to take care of you in your old days was a must, expectations were somewhat low. You could get a lot of kids because making them all share one room was considered fine, they were expected to do a lot of chores, sometimes helping with the family's business even. Contrast that to today: we have financialized everything so you don't need kids, you just need someone to have them, not you specifically. The expectations are high: they need their own bedroom, you have to constantly be behind them (otherwise some busybody will call the cops for neglect), pay for expensive hobbies, tutor them so they can go to a good college (that you're expected to help pay for). Is it really surprising then that people get less or no kids?
Of course this quality over quantity approach may actually be a good thing, I certainly appreciate having been raised that way, unfortunately we have structured our societies to expect infinite growth so until we find a way to resolve these conflicting needs we will keep feeling the pain.