1) There is an entire family planning bureaucracy within the CCP setup to manage this; would do you do with all those people if they are not needed anymore?
2) China is more strict on population planning for certain minorities - not officially, but in the background; think Muslims in western China or Tibet or Mongolians, or ethnic Koreans in northeastern China... If you had no population planning for all the Han chinese, you might have a brighter light cast on this small in number but impactful population control actions.
3) Once someone has power, why let go of it? The CCP survives by creating a fairly good life for the majority in China - and it has done that extremely well - in exchange for control. It is easier to go from 3 back to 2, than it is to go from unlimited back to 2, for example.