A lot of those girls were hidden or otherwise, kept off the books from the Chinese government. Not actually that hard in the more rural areas. Not that that excuses the Chinese government's polices, but the families of those girls weren't all that depraved. Lot of of adult women suddenly started materializing out of nothing for college.

It's been years, so citing my source is hard, but I'll see what I can find.

edit: https://news.ku.edu/news/article/2016/11/22/study-finds-chinas-missing-girls-theory-likely-far-overblown

...have analyzed statistics and found that a combination of late registration and unreported births explains a larger portion of the "missing girls" than previously reported in Chinese sex-ratio-at-birth statistics. Their findings are published this month in the journal China Quarterly.

The researchers believe local government officials informally worked with farmers and acknowledged that they couldn't fully enforce the one-child policy. Instead they made tacit agreements in allowing families to have extra children in exchange for social stability in their communities. The cadres, or local governments, would then under-report “out of plan” births that ultimately influenced the national population statistics.

There was also abortions happening and infanticide probably trails behind that. Not great, but not as bad as is passed around. People probably aren't having kids in China for the same reason they're not having kids everywhere else. Despite what we hear, people in China are either too poor or too busy with work.