Sometimes when aliens visit earth, they have a limited amount of time and have to make choices about which humans to abduct and perform experiments on, and it can be difficult to realize that they might not get a chance to probe every specimen they want to.
Overpopulation is a problem that extends even beyond the overpopulated planet. Let's make things easier for these aliens to do everything they want to us, okay?
If you have fewer children, there's less chance that your children will be abducted and probed. It's math!