Many animals breed less in captivity. Many do not breed at all. The less people feel a sense of control over their own lives, the weaker the desire to procreate becomes. Even if people use birth control, sex drive is still ultimately a reproductive urge. If you're a farmer in the 19th century ? You have a great deal of freedom and independence. Mid 20th century American worker? Homes are cheap. Jobs are abundant. Labor rights are strong. You have lots of options on how to earn a living. You have a clear path to getting a home and raising a family on your own terms. Present-day 20 something? You don't just have roommates in an apartment; you're actually sharing bedrooms. Or you're stuck living under your parents' roof forever, your only hope of home ownership being them passing it on to you. All the jobs available to you don't come close to providing any kind of independent living, let alone the wages to actually buy a home, start a family, etc. You're just going to have to run on the wage slave treadmill til you die.

The economy has developed to squeeze ever more out of workers and consumers. We're psychologically manipulated at every turn. Every business feels like a scam trying to pull one over on you. You can't even sign up for a gym membership without risking getting caught in dark business practices (their infamous cancellation policies.) People feel and are trapped in a system that they have little to no control over. Politicians only listen to the wealthy. Unions provided some voice to the voiceless for some time, but those have been gutted. People feel like cornered animals. They feel as though the entire system is just on elaborate exercise in thinly veiled slavery meant to extract every last once of the work product of your entire life from you. The wealthy pay you a pittance for your labor and then take all that pittance back in usurious rents. People feel trapped.

Lots of animals refuse to reproduce in captivity. Lots of animals reduce their birth rates under captivity. Why should humans be any different? Why should we be any different than the mice of Universe 25?